On April 6, 2017, the United States bombed a Syrian airbase. The bombing was in response to a chemical attack immediately thought to be ordered by the behest of Syria’s President Bashar al-Assad just two days before.

This bombing was illegal. Even the U.S. government has since repeatedly failed to legally justify it.

With the war in Syria now in a new phase, understanding why is more important than ever. ISIS is all but wiped out. The U.S. mission in Syria has become an open-ended commitment involving, among other things, regime change. The U.S. military is increasingly threatening Syria and targeting its allies. In this new phase, grasping why last year’s bombing was illegal will be useful if President Trump plans a repeat.

First, the bombing was illegal under international law because it violated the U.N. Charter. As a signatory, the United States cannot violate Syria’s "territorial integrity or political independence" unless in self-defense, per Article 51, or without Security Council authorization, per Article 2(4). Neither held here.

To fit Article 51, the Pentagon might’ve cited "collective self-defense," which it has used to justify occasional offensive strikes against Syrian forces near U.S. bases or allies. This isn’t patently absurd. (Only a little absurd: unlike the usual self-defense argument, this one involves the "victim" invading the "aggressor.") Regardless, the rationale last April was not self-defense, but to "deter the regime from using chemical weapons again."

At best, we’d have a humanitarian justification, which is not mentioned in the U.N. Charter. Humanitarian intervention is also not sufficiently established to be customary international law. And it’s undermined by ongoing U.S. military, resource, and rhetorical support for Saudi Arabia’s humanitarian catastrophe in Yemen.

What of domestic U.S. legal bodies? The April strikes were also illegal under domestic law because the U.N. Charter is a U.S.-ratified treaty.

Our own constitution’s Article VI says such treaties are "supreme law of the land." To override the treaty prohibition, Congress would have to pass a statute authorizing the attack.

Contrary to the administration’s rationale, the 2001 Authorization for Use of Military Force (AUMF) is such a statute, but doesn’t apply here because Syria is neither al Qaeda nor an "associated force."

The War Powers Resolution of 1973 (WPR) also couldn’t justify the strikes. The WPR gives the president power to initiate war without Congress only for defensive reasons (section 2(c)). And other sections unequivocally avoid "granting any authority to the President," which "he would not have had in the absence of this joint resolution."

To be sure, defenders have their strongest argument in the U.S. Constitution. But even there, the strikes were unconstitutional.

The Constitution only grants to the president the power as commander in chief to "suppress insurrections and repel invasions." Yet without an insurrection or invasion, Congress has the power to declare war, and the Framers intended this power to be Congress’ alone: "All legislative powers herein granted [including to declare war] shall be vested" in Congress.

On the other hand, some defenders of broad presidential war powers brazenly consider the declare-war power historically obsolete–precisely because the president may initiate war, as in the case of Syria.

For support, these defenders sometimes rely on a constitutional drafting quirk. True, the Framers substituted Congress’s "make" war power in an early draft with the "declare" war power, but in doing so they didn’t remove the make-war power. They distributed it among the branches: Congress makes war by initiating war (thus, the declare-war power); and the president makes war by carrying on war (thus, the commander in chief power). An alternative reading endowing the presidency with the power to initiate war is not only inconsistent with, but eviscerates, Congress’s declare-war power.

Happily, no clause in the Constitution is left unexplained when Congress alone may initiate war in the absence of an invasion.

Coincidentally, the Founding generation fully agrees. George Washington refused to attack hostile Indian tribes because "The Constitution vests the power of declaring war with Congress; therefore no offensive expedition of importance can be undertaken until after they shall have deliberated upon the subject, and authorized such a measure." Alexander Hamilton, the great defender of a strong executive, said, "[The Constitution] has only provided affirmatively, that, ‘The Congress shall have power to declare war’…in other words, it belongs to Congress only, to go to War." Other concurring luminaries included James Madison, John Marshall, James Wilson, Thomas Jefferson, John Adams, and James Monroe, among others. Scholars cannot find a prominent early American maintaining the opposite view.

In context, this should make sense: to "declare" war in the 1700s meant to initiate war by a public act, whether by "formal declaration or by commencing armed hostilities."

Could an advocate of presidential war-initiating still rely on the practice of later generations? No. Unconstitutional acts by later Presidents do not make their acts any less unconstitutional.

No matter how you slice it, last April’s airstrikes were illegal. The only unanswered question for this new phase in Syria’s war is whether that will matter if the United States conducts a redo.

Michael Zigismund is a practicing attorney and a Young Voices Advocate. He holds a J.D. from Benjamin N. Cardozo School of Law, and a B.A. from Tufts University in International Relations and Political Science.