The leader of Iran's powerful Islamic Revolutionary Guard Corps Quds Force, Qassem Soleimani was killed in a U.S. drone strike last week. Iran has vowed retaliation against the U.S. The last few weeks have seen tension flair between Iran and the U.S.

Earlier on Dec. 27, 2019, a barrage of more than 30 rockets were fired at an Iraqi military base near Kirkuk, killing an American contractor and wounding four American and Iraqi servicemen. The U.S. accused the Iran-backed militia group Hezbollah for the attack. The militia group, however, denied responsibility.

In retaliation, the U.S. launched a series of airstrikes in Syria and Iraq, killing 25 members of Hezbollah. In response, the militia stormed the U.S. Embassy in Baghdad, the largest embassy of any nation covering 104 acres, just six acres shy of the Vatican. The reception area of the embassy was burnt, though no casualties were reported. Trump, in the following days, in his characteristic style, threatened Iran but it was not really taken to be anything more than an idle threat because he has done so with countries in the past before backing off.

The declaration of war

He threatened Iran after it shot down a U.S. drone but didn't follow through on that threat. The drone strike killing Soleimani came as a surprise to many. Opinions will vary, but the fact remains that this is a declaration of war. Robert Malley, the president and chief executive of the International Crisis Group, said, "Whether President Trump intended it or not, it is, for all practical purposes, a declaration of war."

Opinions are galore. Rhetoric and saber-rattling in abundance. However, was this U.S. strike legal?

The UN codes

Article 2(4) of the U.N. charter says:

"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."

Threat or the use of force is forbidden. However, Article 51 of the U.N. charter allows for the use of force in self-defense. Article 51 says:

"Nothing in the present Charter shall impair the inherent right of individual or collective self-defense 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-defense 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."

But Article 51 deals with a self-defense attack when an armed attack has happened against a state. What does the international customary law say about a situation when one state has to preemptively attack another in order to thwart an imminent threat? When an attack has not occurred. In such a situation, Article 51 is not permissive. That is where the Caroline Test comes into action. The use of anticipatory force or preemptive attack is allowed in international law if the situation passes the Caroline Test, which has four requirements: