Why declare war? Because of the U.S. Constitution. Yes, I am quite aware that only me and probably about a dozen other people still care about that document. And, yes, I'm old, can be cantankerous, and I yell at clouds.
You misunderstand that document, or rather, at least what a declaration of war means. The use or placement of U.S. troops or the lending of war materiel does not require a declaration of war. For all of Western history where official declarations of war were practiced, this was not assumed:
Great Britain did not "declare War" on the Thirteen Colonies during the Revolution
The Dutch did not "declare War" on Great Britain before they sold weapons and gunpowder to the Revolutionaries.
France did not "declare war" on Britain before they permitted American privateers and blockade runners to dock at French ports for repair.
Britain
DID threaten "declarations of war" with both countries if they did not cease and desist. They instructed their ambassadors/diplomats to deliver the messages.
This is the historical background behind the thinking of the framers of the Constitution.
The U.S. Congress never issued declarations of war to any Indigenous peoples it fought with. It never would have assumed it meaningful or necessary.
The United States never "declared war" on the Confederacy.
According to the logic of many here: it clearly should have.
According to the logic of many here: the "lend/lease" program should have been preempted by a declaration of war by Congress.
The United States never "declared war" on the barbary pirates.
Many who claim to cherish the Constitution insist we saddle ourselves with declarations of war in absurd circumstances.
This is a nonsensical standard which no nation in Western history has ever intended by Declarations of War.
A declaration of war is an historical and legal diplomatic term of art signifying a
specific change in international relations.
It is not required to declare war
any time The U.S. (or any world power frankly) deploys military units, troops, or supports with materiel another power.
I do not say this because I disagree with the prospect of leaving Ukraine to the Ukrainians or simply staying neutral in the fighting between Russia and Ukraine (where neither nation has "declared war" to my knowledge. I have no particular opinion accept that I lean towards leaving it to themselves to figure out.
I only say this because the "CONGRESS MUST DECLAR WAR"! crowd has been, for decades at least, misunderstanding what that meant to the framers of the Constitution and their intentions behind it.