When the Constitution was adopted, international law required declarations only for offensive, not defensive, conflicts.
Does the Constitution require a declaration of war for President Trump to maintain his Iran offensive?
Answer: It depends.
Contrary to commonly expressed belief—including misstatements by some constitutional commentators— a declaration of war is not necessarily required for the U.S. to engage in war.
When the Constitution was adopted (as is true today), international law divided just wars into offensive and defensive categories. Offensive wars were designed to punish an adversary for injurious conduct, or to deter, or to obtain compensation for wrongs inflicted. Defensive wars were launched in response to insurrection or invasion, or preemptively to prevent the same. Under the division of powers in the Constitution, the federal government may engage in either kind of conflict, but states may conduct only defensive operations.
Declarations of war, while sometimes issued in defensive conflicts, were required by international law only for offensive ones. Since the Constitution’s legal terms were shaped by the law of the time, the Constitution requires a declaration only in the case of offensive war. In cases of defensive hostility, no declaration is necessary for the President or any threatened state to respond militarily.
Whether the Iran conflict is defensive or offensive depends on whether the Trump administration took action to prevent an imminent attack on U.S. territory or facilities. The administration suggests “yes,” and certainly it is true that during the 47 years of its existence, the current regime in Iran has repeatedly attacked, imprisoned, and killed Americans. There has been some suggestion that the risk to Americans was heightened by prospective Israeli operations against Iran.
Still, as of this writing, we don’t have enough information to determine whether this war is defensive or offensive. If it is offensive, it still may be just, but the Constitution would require a congressional declaration.
Incidentally, a proper declaration of war need not be titled as such. It can be another form of congressional authorization. Thus, the 1964 Tonkin Gulf Resolution, authorizing offensive operations in Vietnam; the 2001 Authorization for Use of Military Force; and the 2002 Authorization for Use of Military Force Against Iraq all qualified as “declarations of war” for constitutional purposes.
