Not just U.S.: You conveniently forget that the entire EU save Hungary--plus Canada, Japan, Australia--has provided money and equipment, and lots of it. Not as much as the U.S., but a huge amount of materiel. Poland, to name one country, has been HUGELY supportive. And all are doing it because the Ukrainians want to defend their country--as they should. Europe does not want a thug/terrorist expanding his footprint, and neither does the U.S. The West did little when Putin stole Crimea from Ukraine, and look what Putin has done. If the West can help Ukraine punch Putin in the face, we should be all for it. it benefits the West. Not to mention that an overwhelming majority of Ukrainians want to align with Europe--not Putin/Russia. Why would they? Ukraine has the potential to be a very successful country--it's big and it's people are quite bright and capable.
As for Putin being "largely restrained" in response to U.S./Western involvement, he has no options save for starting a nuke war. And I don't think
he's foolish enough to do that.