International Legal Standards for the Determination of a State of War

In modern international law, the determination of whether a nation is officially in a "status of war" has transitioned from a rigid, formalistic requirement of verbal or written declarations to a fact-based analysis of hostilities. Historically, a state of war was a legal status that could only be initiated through a specific, performative act—a formal declaration. This act served as a "reasoned declaration of war" or an "ultimatum with conditional declaration of war," as codified in the early 20th century.[1] However, since the adoption of the United Nations Charter in 1945, the focus has shifted from the formal "state of war" to the broader concept of "armed conflict." This shift was intended to ensure that the protections of international humanitarian law (IHL) apply regardless of whether a government admits a legal state of war exists.[2]

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Formal Requirements and the Hague Convention (III)

The primary international treaty governing the opening of hostilities is the Hague Convention (III) of 1907. Under Article 1 of this convention, signatory powers recognize that hostilities must not commence without "previous and explicit warning."[1] [3] This warning must take one of two forms:

1. A Reasoned Declaration of War: A formal document stating the motives for the recourse to arms.
2. An Ultimatum: A conditional declaration of war that stipulates war will begin if certain demands are not met within a specific timeframe.[4]

Furthermore, Article 2 of the Hague Convention (III) requires that the existence of a state of war be notified to neutral powers without delay.[1] While these rules remain technically in force, they are largely considered to have fallen into disuse (desuetude) in contemporary practice. Most modern conflicts begin without such formalities, yet they still trigger the legal obligations of the Geneva Conventions.[5]

The Shift to "Armed Conflict" under the UN Charter

The United Nations Charter fundamentally altered the policy for determining a status of war by prohibiting the "threat or use of force" in international relations under Article 2(4).[6] Because the Charter generally outlaws war, states have become reluctant to issue formal declarations, as doing so might be construed as an admission of an illegal act of aggression.[2] [7]

Consequently, international law now distinguishes between a "declaration of war" (the formal statement) and a "state of war" (the factual reality). Under Common Article 2 of the 1949 Geneva Conventions, the laws of war apply to:

• All cases of declared war.
• Any other armed conflict which may arise between two or more High Contracting Parties, even if the state of war is not recognized by one of them.[8] [9]

This means that the "policy" determining the status is now objective. If a state's military forces engage in hostilities against another state, a "state of war" (or international armed conflict) exists as a matter of law, regardless of whether any official filings or statements were made.[10]

Domestic Procedures and Constitutional Filings

While international law emphasizes factual hostilities, domestic law often still requires specific "filings" or legislative acts to trigger internal war powers.

• United States: Under Article I, Section 8 of the U.S. Constitution, only Congress has the power to declare war.[11] This is typically done through a Joint Resolution. However, the War Powers Resolution of 1973 allows the President to engage in hostilities for limited periods without a formal declaration, provided they report to Congress within 48 hours.[[12]]
• Commonwealth Realms: In nations like the United Kingdom, Australia, and Canada, the power to declare war is a Royal Prerogative, exercised by the monarch or their representative (Governor-General) on the advice of the Prime Minister.[1] [13]
• Other Nations: Many modern constitutions (e.g., Brazil, France, Netherlands) require the executive to seek prior approval or subsequent ratification from the national legislature to officially recognize a state of war.[1]

Necessary Official Statements and Recognition of Belligerency

In some instances, a state may issue a statement that is not a declaration of war but functions as a "recognition of a state of war." For example, Israel’s invocation of Article 40(a) of its Basic Law in 2023 was described as a formal declaration of a "state of war" to trigger domestic military authorities, even though the adversary was a non-state actor.[14]

In the 19th century, a specific policy known as Recognition of Belligerency was used. If a state officially recognized a rebel group as a "belligerent," it signaled to the world that a state of war existed, thereby invoking the laws of neutrality for third-party nations.[14] [15] Today, such official statements are rare, as nations prefer terms like "Special Military Operation" or "Police Action" to avoid the legal and political ramifications of the word "war."[1] [16]