It was obvious during his 4 years in the White House that Trump was trying to ease the US out of NATO commitments, to the point where the European NATO countries had no faith in him. In his speeches he deleted the sentence his speech writers had where it stated the US commitment to NATO article 5, wherein an attack on one member was an attack on all. When Trump was president he always complained about the sanctions that Congress placed on Russia, and always slow played or tried not to apply them.
I'm sure Putin saw Trump as a useful idiot, and his dick's been in Trump's mouth or Trump's tongue up Putin's ass since 2016. Just look at how quickly Trump jumped last week praising Putin. Not a surprise given that his business was based on investment from Russia money laundering, and he may still be waiting for the golden shower video to be released.