The U.S. wanted to give them weapons to defend themselves but not to the point of giving IRBMs or Tomahawk missiles, something that Russia could use as a very good excuse to invade. However, Putin just using the words Nazis and Ukraine shouldn't have exist anyways was enough to invade the country.Ukrainian Top brass and/or President is an idiot.
#1. They never believed Russians are serious about Invasion despite so many warnings.
#2. They were hoping that rest of the NATO will support them, while all they US/UK did just sent them small arms and infantry weapons for guerrilla warfare instead of serious weapons. Which meant US and UK aren’t planning to defend Ukraine instead they wanted to use Ukraine as a burning pot for Russian army.