I trust him to listen to the joint chiefs, and our intelligence services, as far as our security. When Russia came over the boarder to take more territory, he made the right decision to back Ukraine vying to maintain it's independence, as there is no telling what area or country (former Soviet block or not) they would want to move to next. I think Putin made a huge mistake doing, thinking NATO countries would not support Ukraine with resources to protect itself, as the next territory, state or country might be closer to NATO or part of European Union. Now you see other countries that see what Russia is doing, though previously not NATO countries, applying for NATO membership to insure their own freedom from Russian aggression.
Back in my Dad's day, we sat on our hands while European countries were facing aggression by the NAZIS. We don't want to have to fight our way back on to that continent, when we could have supported the first country to be attacked, united NATO against the thread and avoided the broader war that would kill millions.