12L - Use of force

History

War as an expression of State sovereignty

• ‘no state can judge another state’

Covenant of the League of Nations

• did not try to prohibit war, just regulate it

Kellog-Briand Pact 1928

• renounce war as an instrument of national policy

• substantive prohibition of war

Charter of the UN

Prohibition

• Article 2(4) “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations

The prohibition of the use of armed force

Prohibition of the threat and use of force

• Force: armed/military

  • Force: it means armed force or military attack (any method of imposing your will on another State against their will (can include all kinds of force such as economic coercion, general political intimidation).

• IR: between States, not within a State

  • Within the state they can use force

• Use and threat

  • depends, but threat is kind of prohibited as well

• Absolute (comprehensive)

  • no loop holes

• Customary IL

  • prohibits the use of force just as much as the Charter

• Jus cogens (alternatively: prohibition of aggression)

  • prohibition of use of force is a jus cogens

The right of self-defence - UN Charter

Article 51 Charter: “Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.”

• customary IL: no need to report

• then if no report maybe it is not convincing to use self-defence?

The right of self-defence - sources, meanings and conditions

Sources

• article 51 Charter: treaty law

• customary international law

  • ICJ, Nicaragua case, para. 176

  • right of self-defence against armed attack

  • requirements of necessity and proportionality

Meaning

• self-defence = use of armed force in response to armed attack

• armed attack: unlawful use of armed force

• armed force: prohibited in international relations

  • any use of armed force by a State on and/or against the territory of another State is prohibited

  • no self-defence against lawful use of force (e.g., no self-defence of Iraq against coalition authorized by SC in Resolution 678 (1990))

  • only against unlawful armed attack

• self-defence: legal justification for armed action on the territory of another State or against the armed forces of another State(i.e., against military targets)

Conditions

• armed attack: conditio sine qua non

• against a member (under customary international law: all States have the right of self-defence)

• measures taken in self-defence are to be immediately reported to the Security Council

• State must declare itself to be under attack

• request for assistance → collective self-defence

• Nicaragua case, paras. 195, 196-199, 200; Armed Activities case (Congo v. Uganda), para. 146

Conditions - armed attack

• Under customary IL

  • Necessity (+immediacy - attacked, immediate response)

  • Proportionality

    • achieving the goals of self-defence

• Case: invasion Falklands by Argentina

  • 2-3 weeks for UK to reach island

  • must be judged based on the circumstances

The right of self-defence - armed attack

Interpretation in context

• agression armée - French text article 51 Charter

• article 2(4) Charter: use of force

• article 51 Charter: armed attack

the gap → implication of different terms

an armed attack = use of force (more restrictive)

all uses of force ≠ armed attack

• less grave uses of force = use of force

• most grave uses of force = armed attacks

  • Nicaragua case, para. 191

Interpretation in context - the Court went on

• armed attack by regular armed forces across an international boundary

• armed force by armed bands (etc.), sent by or on behalf of a State, or its substantial involvement therein

• armed attack ≠ mere frontier incident

• armed attack ≠ assistance to rebels:provision of weapons, logistical support, other support (but may be a use of force)

• force by armed bands scale and effect similar to attack by regular armed forces

• Nicaragua case, para. 195

  • article 3(g) Definition of Aggression: GA Resolution 3314

Russia-Ukraine War (2014 - )

Taking over and annexing Crimea, military support and participation in war in Donetsk + Luhansk (2014)

Full scale invasion, annexing Donetsk, Luhansk, Zaporizhia and Kherson (2022-...)

Thailand-Cambodia conflict (2025)

Frontier incidents

Conscious attacks

> difficult to say who started it

Oil platform case

paras. 51 and 72

• affirmation Nicaragua case

• Mining of single military vessels as an armed attack (?)

  • could constitute, depends what happens after

Armed activities by non-state actors

article 51 Charter: no mention of origin > opens interpretation

• case law - relatively restrictive

  • Nicaragua case, para. 195: sending (by or on behalf of a State) of armed bands to territory of another State, or its substantial involvement therein

  • Oil Platforms case, paras. 57-61: burden of proof, US could not prove

  • Wall opinion, para. 139: self-defence by a State against armed attack of another State; attacks are not claimed to be imputable (attributable) to a foreign State

  • Armed Activities case (Congo v. Uganda), paras. 146-147

    • no sending by or on behalf of Congo

    • no involvement Congo

    • no need to determine the question of self-defence against large-scale attacks by irregular forces (para. 147)

Standard

• no involvement of a State required in any sense?

• involvement State

  • attribution (esp. based on article 8 ARSIWA: conduct under instruction, direction or control of a State)

  • substantial involvement: involvement of a State in armed activities of nonstate actors (e.g., in planning, coordinating, military supporting)

  • support and/or harbour (Nicaragua case, para. 195, does not support this)

  • unable or unwilling doctrine - unwilling/unable to repress

Armed activities by non-state actors - US strikes on drug-boats (2025)

US strikes on drugboats (2025)

• drug cartels designated as foreign terrorist organizations (FTOs)

• drug cartels as FTOs commit armed attack against US

• fentanyl as WMD (weapons of mass destruction):

  • “illicit fentanyl is closer to a chemical weapon than a narcotic,” and that its manufacture and distribution “threatens our national security and fuels lawlessness in our hemisphere and at our borders.”

Armed activities by non-state actors - Hamas attack on Israel

Western States: Israel has the right to defend itself

State to State: articles 2(4) and 51 Charter - not in this way, Palestine not recognised as State

• if Palestine is a State, the prohibition of the use of force and the right to self-defence (under customary international law) are applicable to both Palestine and Israel

• a complication in that case is that Israel occupies (part of) the territory of Palestine: this may allow for an argument that Palestine has the right to self-defence (see: article 3(a) Definition of Aggression, Resolution 3314

• → ICJ, Legal Consequences arising from the Policies and Practices of Israel in the Occupied Palestinian Territory, including East Jerusalem: the continued presence of Israel in the Occupied Palestinian Territory is unlawful

State to non-state actor

• Israel blockades Gaza

• it is alleged that Iran supports Hamas, but concrete evidence appears lacking (US): involvement Iran is probably insufficient to trigger a right of self-defence against Iran (see Nicaragua case; Oil Platforms case)

• right to self-determination: right to resistance (by force)?- this appears to be supported, implicitly, by: “The situations referred to in the preceding paragraph include armed conflictsin which peoples are fighting against colonial domination and alien occupationand against racist régimes in the exercise of their right of self-determination, ...”

  • Article 1(4) Additional Protocol I, Geneva Conventions (Israel is not a party)

  • Is this reflective of customary IL?

Anticipatory self-defence

Anticipatory self-defence

• article 51 Charter: if an armed attack occurs ... present tense

• when does an armed attack start? - any cross-border

• imminent armed attacks?

• case law avoids the question; practice limited; opinio juris divided

Preventative military action

Preventive military action

• the 9/11 attacks: US Security Strategy

• preventive action against terrorist groups

• imminent threats of attacks: uncertainty as to timing and location

Preventive military action

• security threats

  • strikes

  • occupation

• US

• Israel

  • Against Syria, even though they clearly stated that there is no such plan

  • Iran Nuclear Weapons, Israel bombed

• “Our President, Donald J. Trump, authorized this operation to neutralize the threat that the Islamic Republic of Iran's nuclear program posed to Israel and to international peace and security. The objective was to destroy Iran's nuclear enrichment capacity and stop the threat that this rogue regime obtains and uses a nuclear weapon. That threat must be considered in light of more than four decades over which the Islamic Republic of Iran has launched unprovoked attacks against the United States and other UN Member States, violated the UN Charter and the norms of civilized nations, materially supported international terrorism, sought to eradicate the State of Israel, and called for “Death to America.”

  • Letter US to Security Council, 27 June 2025

• “Article 51 of the Charter may justify a use of force in self-defence only within the strict confines there laid down. It does not allow the use of force by a State to protect perceived security interests beyond these parameters. Other means are available to a concerned State, including, in particular, recourse to the Security Council.”

  • Armed Activities case, para. 148

  • Security interest are not strong enough to invoke self-defence

Protection of nationals

Protection of nationals

• self-defence

  • population as an essential ingredient of statehood

→ = polulation on the territory of the State

→ ≠ foreign nationals on territory of other States

• separate, self-standing justification under international law?

  • certain practice

  • opinio juris uncertain

Humanitarian intervention / R2P

Humanitarian intervention

• not in Charter

  • (human rights mentioned in articles 1(3) and 55-56 Charter)

  • certain practice; opinio juris divided

• States intervene under self-defence usually

• NATO intervention Kosovo (1999)

Responsibility to Protect (R2P)

• International Commission on Intervention and State Sovereignty(ICISS; 2001)

• concept

  • sovereignty as control → sovereignty as responsibility

  • humanitarian intervention / right to intervene → responsibility to react

  • primary responsibility State

    • → subsidiary responsibility international community

    • → first of all the Security Council,

    • then General Assembly, regional organizations, States

• concept

  • large scale loss of life, actual or apprehended (with / without genocidal intent)(deliberate State action or neglect or inability to act, or a failed State)

    • genocide

    • internal war

    • insurgencies

    • crimes against humanity

    • war crimes

    • repression

    • State collapse: mass starvation / civil war - natural or environmental catastrophes, with State unwilling or unable to act (earthquakes, tsunamis, etc.)

• large scale ethnic cleansing, actual or apprehended

World Summit Outcome (2005): GA A/RES/60/1

• genocide

• crimes against humanity

• war crimes

• ethnic cleansing

• “138. Each individual State has the responsibility to protect its populations from genocide, war crimes, ethnic cleansing and crimes against humanity. This responsibility entails the prevention of such crimes, including their incitement, through appropriate and necessary means. We accept that responsibility and will act in accordance with it. The international community should, as appropriate, encourage and help States to exercise this responsibility and support the United Nations in establishing an early warning capability.”

  • World Summit Outcome (2005): GA Resolution A/RES/60/1

• “139. The international community, through the United Nations, also has the responsibility to use appropriate diplomatic, humanitarian and other peaceful means, in accordance with Chapters VI and VIII of the Charter, to help to protect populations from genocide, war crimes, ethnic cleansing and crimes against humanity. In this context, we are prepared to take collective action, in a timely and decisive manner, through the Security Council, in accordance with the Charter, including Chapter VII, on a case-by-case basis and in cooperation with relevant regional organizations as appropriate, should peaceful means be inadequate and national authorities are manifestly failing to protect their populations from genocide, war crimes, ethnic cleansing and crimes against humanity.”

  • World Summit Outcome (2005): GA Resolution A/RES/60/1

  • Security Council - veto powers

+elements of Article 2(4) of the UN Charter

Force

• Core idea: Article 2(4) prohibits the use of force in international relations.

• War: Historical sources like the Covenant of the League of Nations, Kellogg-Briand Pact, and Corfu Channel Case help define “force” legally.

• Ordinary meaning debate: Scholars ask what “force” actually includes:

• Armed/military force: traditional armies, invasions, attacks.

• Direct/indirect: must it be actual military action or also threats, coercion, or proxy actions?

• Economic/political coercion: does it cover sanctions, blockades, or other non-military pressure? (debated)

2. Territorial Integrity

• Ordinary meaning: Refers to a state’s territorial boundaries being respected. Questions include:

  • Is the principle about inviolability of borders?

  • Does it only prohibit loss of territory or annexation?

• Object and purpose / subsequent practice:

  • UN practice and resolutions clarify meaning over time:

    • A/RES/2625: Acquisition of territory by force is illegal.

    • A/RES/3314: Provides a formal definition of aggression.

• Essentially, this protects states from other states taking land by force.

3. Political Independence

• Ordinary meaning: States should be politically free from domination.

• Existing/emerging states: Covers both established states and new states seeking independence. This links to:

  • Self-determination: people’s right to choose their government without external interference.

  • Reference to art. 10 League of Nations Covenant: Early recognition of states’ rights to independence and self-determination.

+Humanitarian intervention & R2P

Humanitarian intervention:

• Coercive action by States involving the use of armed force in another State

• Without the consent of its government

• Without authorisation form the UNSC

• For the purpose of preventing or putting to halt gross and massive violations of human rights or international humanitarian law

> A moral/legal right when the UNSC is blocked from acting?

> Kosovo crisis - NATO bombing

> Is there a legal basis?

R2P

• Primary responsibility of States: States have the responsibility to protect their own population from genocide, war crimes, crimes against humanity, ethnic cleansing

• Responsibility to assist: International community has the responsibility to assist a State in fulfilling its R2P

• Responsibility to act: International Community has the responsibility to take timely and decisive action by using UN procedure

R2P:

Preventative nature: use of force as the last resort

Specific set of crimes

Use of force through the UN channels

Internal and protective focus

Humanitarian intervention:

Use of force as the primary means

Any gross violation of human rights

Additional exception?

External and interventionist focus