Thirty years ago US, Britain and Russia signed a deal with Ukraine that they will protect them in exchange for their nuclear weapons.
www.bbc.com
This is an agreement US and UK went into because a poor country like Ukraine having these weapons could be unsecure and evetually end up in a bad actors hands( North Korea, Al Queda, Iran....)...
It was the US that was pushing the deal... US wanted that security for itself as they predicted that they would be the main aim of those bad actors... There was no metion or expectation of Ukraine joining or not join EU or NATO.
en.wikipedia.org
So Russia then decides to invade Ukraine in 2014... The US and EU knowing their obligations to the deal sanctioned Ukraine, then Russia invaded Crimea, the sanctions increased and US/EU started to arm and train Ukraine to better protect themselves ....
Russia then went for the whole country. Ukraine repealed them and a three year war started...
Now Trump wants to renege on the whole deal, he is basically saying that Ukraine were fools to sign a deal with America because US doesn't honour deals any more...
As with any deal, if you back up, you have to give them there nuclear weapons back...
US loosing trust in the world severely weakens its globally standing. Hard to do business if there is no trust.
US got what they wanted and Ukraine can basically goto hell..

Ukraine gave up its nuclear weapons in 1994. Now it's asking why
It's 30 years since Ukraine agreed to give up its nuclear arsenal. Some now question why they did.

This is an agreement US and UK went into because a poor country like Ukraine having these weapons could be unsecure and evetually end up in a bad actors hands( North Korea, Al Queda, Iran....)...
It was the US that was pushing the deal... US wanted that security for itself as they predicted that they would be the main aim of those bad actors... There was no metion or expectation of Ukraine joining or not join EU or NATO.

Budapest Memorandum - Wikipedia
So Russia then decides to invade Ukraine in 2014... The US and EU knowing their obligations to the deal sanctioned Ukraine, then Russia invaded Crimea, the sanctions increased and US/EU started to arm and train Ukraine to better protect themselves ....
Russia then went for the whole country. Ukraine repealed them and a three year war started...
Now Trump wants to renege on the whole deal, he is basically saying that Ukraine were fools to sign a deal with America because US doesn't honour deals any more...
As with any deal, if you back up, you have to give them there nuclear weapons back...
US loosing trust in the world severely weakens its globally standing. Hard to do business if there is no trust.
US got what they wanted and Ukraine can basically goto hell..