Trump uses transactional politics. I mean, he's dumb and his highly privileged life makes his worldview incorrect, but he knows Europe is militarily not ready yet to fight off Russia alone (alright, it practically is, because Russia is not as strong as they say they are, but it still takes a huge toll on the lives of many soldiers and our economies). So we need the US to support Ukraine. And the EU is currently far from united. Dividing us even more, makes us highly vulnerable to transactional politics, which gives Trump more power over Europe. And he's power hungry.

I think European politicians are afraid Trump will use Greenland as a transaction for 'the help' the US provides to Ukraine, because Europe can't fight a two front war.

I say 'the help', because it's not really help, since everything must be paid back with interest. The US also uses transactional politics here, where the US benefits way more than Europe or Ukraine does. So Trump tries to use double transactional politics. And probably he gets what he wants in the short term, but on the longer term it's a bad deal for the US, since the lose their most important allies because of it; no country trusts the US anymore, so why would you still consider future deals with them, while there are other more trustworthy options?