Does no one remember how this game is played? Of course the U.S. can’t send troops in against Russia. Of course it can’t threaten military action. But there are lots of things the U.S. can do to put the squeeze on Russia and remind them that this kind of behavior is not rewarded.
For example, you make a statement that in light of recent events you are calling an emergency meeting of NATO to determine if any of the Baltic states need additional military support. Last year the U.S. placed its first continuously-stationed military unit in Poland. Poland and the U.S. can announce negotiations to seriously beef up U.S. military presence there. The U.S. and Poland can announce negotiations to sell Poland advanced military hardware (they don’t have to actually do it - just credibly threaten to do so).
The U.S. can announce that in light of recent Russian moves, it is reconsidering the expansion of missile defense bases in Europe.
If Ukraine agrees, the U.S. can move ‘advisers’ into the country to help ensure that the gas pipelines are protected and to verify Russian military movements. In fact, the adivsers are a tripwire - if Russia expands military operations it risks killing American soldiers, which could escalate the situation.
Then there are economic sanctions. Russia depends on a lot of western imports. And it needs to export its natural gas. Announce that in light of Russian aggression, the U.S. is setting up negotiations with the EU to consider alternate pipeline routes to move energy into Europe in case Russia starts playing hardball with Europe’s energy. In fact, what it is is an economic threat to Russia’s exports.
Strategically targeted tariffs that hurt Putin’s oligarch allies can put pressure on him. Increase Russian export restrictions on militarily sensitive equipment, and expand the list to include things that will hurt the country economically as well. Russian bank accounts in the west can be frozen.
Putin has announced that he is going to build military bases in a number of countries around the world. The U.S. can begin to use whatever leverage it has with any of those countries to put the screws to Putin’s plans.
That’s how hard diplomacy is done. There are a million little levers you can push and pull to make your opponent’s life uncomfortable without threatening military action. That was how negotiations worked throughout the cold war - often behind the scenes. The U.S. employed the same strategy with Taiwan - Every time China would get frisky, the U.S. would offer to sell ever-more advanced weapons to Taiwan. They made it clear to China that Taiwan would be defended, and furthermore that it would be armed in proportion to China’s aggression against it, so that it could defend itself even if the U.S. wasn’t there.
You don’t have to go to war - you just have to make it clear that you’re not going to put up with Putin’s aggressive stance, and if he keeps it up he’ll find himself slowly strangled economically and outmatched militarily. Russia has nowhere near the economic power of the U.S., and cannot withstand much economic pressure.
Doing nothing is how these things escalate. Saddam invaded Kuwait because he thought the U.S. would do nothing. Hitler invaded Poland because the response to his first aggressions made him think he could do what he wanted without reprisal. Japan thought that if it bloodied the U.S.'s nose hard enough the U.S. would negotiate a peace, and part of the reason it thought so was because of the loud isolationist movement in the U.S. and the refusal of the U.S. to enter the war in Europe.
A little pushback against early minor provocations can prevent the necessity of major pushback when the provocations finally become too great to ignore.