The two disagreed on many things but were able to live with each other.
But Putin himself has changed since Trump's first term.
Former British Prime Minister Tony Blair has said the Putin today is almost unrecognizable from the man he knew in the 2000s.
As Trump is discovering, Putin also is very different from the man he dealt with previously.
Trump said in a social media post Sunday, "I've always had a very good relationship with Vladimir Putin of Russia, but something has happened to him."
It is widely believed that Putin was encouraged to invade Ukraine after he saw then-President Joe Biden order America's hasty and humiliating exit from Afghanistan in 2021.