He thinks he can settle the Ukrainian War (actually the Russian invasion of Ukraine). But his first step in the "negotiation" has been a blunder. His sycophants met with Putin's sycophants in Saudi Arabia, and both sides are bragging about what occurred.
But where was the delegation from Ukraine? They weren't there because Trump didn't invite them. Trump doesn't think it's important for Ukraine to have a say in the war raging in their country. He thinks he can sweet talk Putin into accepting his terms. He might be right, because Trump's "terms" are very advantageous to Russia.
To achieve peace, Trump is ready to give Russia about 20% of Ukraine (the eastern provinces they currently control after the invasion). And he would remove the sanctions imposed on Russia. Russia would love that. It would give them the time and money to rebuild their military
Trump mistakenly thinks Putin would be content with his proposal and there would be a lasting peace in the region. He's wrong!
Many people believed the same when Russia invaded and took the Crimea away fro Ukraine. But Putin wasn't satisfied. He then invaded Ukraine. And it wasn't just to get the eastern regions - Putin wanted all of Ukraine - sending his troops and tanks straight for Kiev. He was only stopped because he underestimated the skill and bravery of Ukraine's military. Now he is trying to use bombs and rockets to make Ukraine submit and surrender.
European leaders are not as stupid as Trump. They know Putin's real goal - one he has openly admitted. He wants to bring all of the countries that once were in the old Soviet Union back under his control. Ukraine is just the first step of his dream.
They know that once Putin has rebuilt his military, he will again attack Ukraine. And once defeating it, he will move on to other countries (Latvia, Estonia, Poland, etc.). And when he does, the NATO nations (with or without the U.S.) will have to respond - starting a new World War. The European nations must defend Ukraine to prevent this. Trump egotistical idiocy will not protect them.
Being a bully himself, Trump doesn't understand how to beat one. You don't stop a bully by giving in to his demands. That just results in bigger demands. The only way to stop a bully (like Putin) is to stand up against them with force (if necessary).
Trump thinks he's winning, but he's actually surrendering. He's just not smart enough to know it.