Namely, according to U.S. law, the president has the right to use the American military on a foreign battlefield for up to 60 days without congressional approval. This deadline is expected to expire on May 1, although some analysts in the U.S. believe it expires on April 29. The discrepancy in dates stems from a relatively vague law that does not distinguish between the start of the conflict (which for the U.S. war against Iran was February 28) and the day Congress was notified of the war (which was March 2).
However, the situation is further complicated by the ceasefire currently in effect. Many Republicans, as well as some members of the Democratic Party, do not believe that the ceasefire period, which has now lasted longer than the original two weeks, should be included in the 60-day period prescribed by law.
As for the law itself and its enforcement, Congress has never voted to limit the power held by the U.S. president, despite numerous attempts, including several over the past months. Additionally, U.S. courts, including the Supreme Court, have never ruled on the constitutionality of this law, making it even more uncertain what will exactly happen when the deadline expires.
Regardless of the fact that the law has not been enforced and the courts have not ruled on it, previous administrations have sought creative ways to circumvent it, in order to avoid the situation Trump might find himself in. Ronald Reagan was the first U.S. president who could have come into conflict with the law when he sent Marines to Lebanon as part of a peacekeeping mission. He did not formally notify Congress for nearly a year that the Marines were in Lebanon, until they came under attack, when he authorized the use of force.
As part of an agreement with Congress, Reagan only then entered his permitted 60-day period, and Congress subsequently approved the Marines' operations in Lebanon for the next 18 months. Like Reagan, Obama had a conflict with Congress on this issue in 2011, when Libya was in focus. The then-U.S. president rejected the advice of his lawyers and continued the bombing campaign without congressional approval, but sent one of his aides to a hearing to clarify the decision.
Harold Koh, who was then a high-ranking official at the State Department, said that Congress did not need to be notified because "U.S. troops are not in immediate danger" and because most of the action was conducted remotely, using drones. Trump used the same explanation the previous year when he began his bombing campaign against ships in the Caribbean Sea.
Regardless of previous cases, Trump's war in Iran is unique, and it will be interesting to see whether Congress will react.