according to the constitution how can the united states declare war
Declaration of war by the United States For the United States, Article One, Section Eight of the Constitution says "Congress shall have power to ... declare War." However, that passage provides no specific format for what form legislation must have in order to be considered a "declaration of war" nor does the Constitution itself use this term. In the courts, the United States Court of Appeals for the First Circuit, in Doe v. Bush, said: "[T]he text of the October Resolution itself spells out justifications for a war and frames itself as an 'authorization' of such a war."[1] in effect saying an authorization suffices for declaration and what some may view as a formal Congressional "Declaration of War" was not required by the Constitution.
War Powers Clause Article I, Section 8, Clause 11 of the United States Constitution, sometimes referred to as the War Powers Clause, vests in the Congress the power to declare war, in the following wording: