How to declare war.

The power to declare war belongs to US Congress under the US Constitution. This is clearly stated in Article I, Section 8, Clause 11:

To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;

To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;

I added clause 12 because, as I keep pointing out, the issue of standing armies was important to the Constitutional framework. The founders made it clear that standing armies were the tools, and a sign, of tyranny. 

I'm going to cite once again this precursor to the Second Amendment from the Virginia Bill of Rights of 1776 provides a good idea of what the founders' mindset happened to be:

13. That a well regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defence of a free state; that standing armies, in time of peace, should be avoided, as dangerous to liberty; and that, in all cases, the military should be under strict subordination to, and governed by, the civil power.

This isn't to point out the Militia/Standing Army conflict, which was something the constitution bears out, but to point out that the use of military force requires the consent of congress: not unilateral action by a maniac (tyrant).

The law of the Declare War Clause is unsettled in part because there have been very few judicial decisions interpreting it. In the Prize Cases in 1863, the Supreme Court upheld as a defensive measure President Lincoln’s blockade of the southern states following their attack on Fort Sumter, but was ambiguous as to whether the authority for the blockade came from Article II, from specific statutes Congress had passed in 1795 and 1807, or some combination of both. And in dicta, the Court noted that the President could not begin hostilities without Congress’s approval. Earlier cases, such as Bas v. Tingy (1800), referred generally to Congress’s broad powers over warmaking without giving specific guidance on the President’s power. But in modern times, courts have generally avoided deciding war-initiation cases on the merits, based on rules that limit what types of disputes courts can resolve, such as standing or the political question doctrine. As a result, the precise contours and implications of the Declare War Clause remain unresolved today—leaving resolution of disputes over particular uses of force by the President to the political process. 

The president is allowed to use military force for imminent threats, but I would say that those threats need to be actual and directly upon the territory of the United States. Anything other than that would require Congressional approval.

I also disagree with the concept that any use of troops beyond the territory of the United States is not in violation of this since a "peacekeeping" force can find itself embroiled in a conflict. The founders were clear that one of the reasons they preferred militias to standing armies was that they were local defence forces. That is, they would not be used on "foreign adventures": especially those like the one pursued by Trump.

Trump gives fabulous examples of why this clause needs to be strictly interpreted with consideration  to  international law with his latest adventure being both unconstitutional and in violation of international war.

The United States and Israel were in negotiations with Iran, with Iran basically conceding to the demands when both Israel and the United States attacked Iran: which is a violation of international law. Toss in that it discredits both Israel and the United States, which is why they started this last conflict, but won't be the ones to end it. 

It is long past time that the US military serves as a defence force, not one that keeps the country in forever wars.