The US owes the Ukraine a large national security debt that Trump dishonored with his demands for “dirt”. When the Soviet Union collapsed and Ukraine became independent, Ukraine found itself found itself holding a nuclear arsenal. In fact, for a while, the Ukraine had the third most powerful nuclear force in the world.
In return for Ukraine giving up its nuclear capability, Russia, the UK, the US and the Ukraine entered in 1994 into the Budapest Memorandum on Security Assurances. The parties promised Ukraine that they would respect its independence and sovereignty and its existing borders and reaffirmed their obligation to refrain from the threat or use of force against its territorial integrity or political independence. Everyone understood that this was really an American promise to deter Russia from seeking to reannex the Ukraine (in whole or in part). For a variety of reasons, the agreement was not submitted to the Senate as a binding defense treaty, creating some ambiguity in the US commitment. This made it particularly important that the diplomatic signal-calling be clear.
A lot has happened since 1994 but the gist of that promise remains. Some level of strategic support for the Ukraine has been an element of a bipartisan policy for a quarter of a century--- except now the Ukrainians learn that it is of such little regard to the current Putin-loving, American President, that he would abandon the commitment if he doesn't get his political dirt
Other countries and leaders (think, North Korea, think, Iran) are watching too. This conduct provides extraordinary reinforcement to a common, post-Libya narrative—you give up your nuclear program to the US at your peril. In other words what has happened is not just an oops-I-crossed-the-line electoral peccadillo but a damaging national security betrayal.