Does the President have independent constitutional power to deploy troops to Ukraine? My originalist answer is yes.
First, as a general matter the President has constitutional power as commander-in-chief to deploy troops. Of course, he needs money from Congress to pay for any deployments. A President with a specific, tightly controlled military budget might as a practical matter be unable to make a deployment without asking Congress for additional funds. But the modern military budget is not specific and tightly controlled. I'm sure the current President has enough discretionary funds to pay for a deployment to Ukraine.
Apart from the need for funds, the constitutional limit on the President's power to deploy troops is the declare war clause. As I've argued (in Textualism and War Powers), the original meaning of that clause is that Congress has exclusive power to put the nation in a state of war, whether by issuing a formal pronouncement or by directing an act of war. So the President cannot make a deployment that creates a state of war without Congress' approval. But deploying troops to a friendly country (assuming that country gives permission) does not create a state of war — obviously not with the friendly country, and also not with another country that may oppose or feel threatened by the deployment.
It's true that a deployment may provoke a war. In the present situation a deployment to Ukraine might provoke a war with Russia. But the deployment itself does not create a state of war — if war occurs, it will start with a Russian attack. The deployment itself would create a state of war only if it violates Russian sovereignty, and Russian sovereignty obviously dies not extend to whether Ukraine hosts a U.S. deployment.
This conclusion may seem to give the President too much power over war. The issue dates at least to 1846, when President Polk deployed troops to the north bank of the Rio Grande River, provoking an attack by Mexico that started the Mexican War. Some contemporaries (including Abraham Lincoln, then a congressman) thought Polk violated the Constitution. But (contra Lincoln) the Constitution doesn't say the President cannot act provocatively. Many things a President might do, in the conduct of diplomacy, for example, might provoke a war. A no-provocation rule would be impossible to implement and impose too great a restriction on presidential foreign policy. The Constitution only says the President cannot start a war.
That does not mean, though, that Congress is powerless to limit presidential provocations. As to deployments specifically, as noted above the President can make them only if Congress has already approved enough discretionary funds for the military. It's the modern military budget, as much as the Constitution, that empowers the President to deploy troops to Ukraine (and even in 1846, Polk apparently had enough discretionary funds to send the army to the Rio Grande). If Congress doesn't like provocative deployments, it shouldn't hand the President unrestricted money. Further, even if the President provokes a war, Congress can refuse to fund it (or can fund only defensive measures). Polk didn't have enough discretionary funds to fight the Mexican War so he asked Congress to authorize further spending. If Congress didn't like the war, or the way it started, it could have refused Polk's request; instead, Congress approved enough money for an invasion of northern Mexico, and when that proved inconclusive, it approved additional funds for an attack on Mexico City. So Congress seemed to have little ground to complain about Polk's provocations. And generally, the charge that presidential provocations undermine Congress' declare war power are unfounded: Congress has tools to limit provocations, if it want to use them.
Posted at 6:12 AM