Lesson 3: Principles of International Law: Sources of International Law

Jus ad Bellum vs Jus in Bello

Jus ad Bellum

This means the law governing the decision to go to war.

It includes rules under the United Nations Charter on when force is lawful.

Jus in Bello

This refers to the law governing conduct during war, also known as:

• International Humanitarian Law the law of armed conflict.

Once war begins, the legality of starting the war becomes less relevant. Both sides must follow rules such as:

• protecting civilians not attacking civilian objects protecting wounded soldiers.
• These principles are contained in treaties like the Geneva Conventions.

The Key Problem with Preventive War

The danger is that any state could claim a future threat.

For example:

• A state could argue another country might develop weapons in the future.
• It might then attack before any real attack occurs.

International lawyers worried this would undermine the entire prohibition on the use of force established after World War II.

Nuclear Weapons and the “Deterrence ” Reality In practice, states often believe that possessing nuclear weapons prevents other states from attacking them. This idea is called nuclear deterrence.

Examples often mentioned:

• India and Pakistan both possess nuclear weapons, which discourages full-scale war despite border clashes.
• North Korea developed nuclear weapons partly to deter intervention by powerful states.
• China and India have border disputes but avoid escalation partly due to nuclear capability.
• This is sometimes called the “big boys club”—nuclear-armed states that are harder to attack.
• However, deterrence is a political and strategic concept, not a legal justification under international law.

International Law on Nuclear Weapons

The key treaty regulating nuclear weapons is the:

• Treaty on the Non‑Proliferation of Nuclear Weapons (NPT)

The treaty aims to:

• Prevent the spread of nuclear weapons.
• Promote peaceful nuclear energy.
• Encourage nuclear disarmament.

But there are major problems:

• Some states never joined the treaty.
• Some states possess nuclear weapons but are recognized by nuclear powers.

For example:

• Israel has not signed the NPT.
• India and Pakistan are not parties to the treaty.
• North Korea withdrew from it.
• This weakens the global non-proliferation regime.

The UN’s Role in Controlling Nuclear Weapons

The main international body responsible for nuclear oversight is the:

• International Atomic Energy Agency (IAEA)

Its role includes:

• inspecting nuclear facilities monitoring compliance with the NPT reporting violations to the United Nations Security Council.
• But the UN system has limits.

The Security Council must act collectively, and its five permanent members have veto power:

• United States
• United Kingdom
• France
• Russia
• China

If any of these countries veto a resolution, action may be blocked.

Legality of Attacking a State Over Nuclear Weapons

Under the United Nations Charter, attacking another country because it might develop nuclear weapons is generally illegal.

The only lawful options are:

• Self-defence

Under Article 51, if there is an armed attack.

Collective Security If the United Nations Security Council authorizes force.

Without these conditions, the use of force is usually considered a violation of international law.

Jus ad Bellum vs Jus in Bello (Important for Exams)

Jus ad Bellum

The law governs when states can go to war.

Main rule: prohibition of force under the UN Charter.

Jus in Bello

Also called:

• International Humanitarian Law

This governs how war is conducted, including rules such as:

• protecting civilians banning attacks on civilian objects protecting wounded soldiers.
• These rules come from treaties such as the Geneva Conventions.
• Even if a war is illegal, both sides must still obey IHL.