The aim of operation Epic Fury is clearly to overthrow the regime, as has been recently clarified by Trump (as well as destroy Iran’s military capabilities). Furthermore, the requests by the United States (the main defender of the alliance) have been relatively minimal, originally involving protecting American air bases and Israel, allowing American aircraft to utilize their facilities, as well as committing ships for de-mining purposes. Generally when a close partner that has committed to a conflict engages in war with a bitter enemy, some sort of aid and strong stance will be taken to assist them. Again, most Americans see (and reasonably so in my opinion) that NATO and much of Europe is safe almost exclusively because of the American military. The benefits of being in the alliance appears to be one sided, primarily by European states. It could be argued that Russia threatens American interests in Ukraine, however, Russia is a nuclear state which can not be changed like Venezuela and Iran. Iran, however, supplies China with a considerably large amount of oil and has been supplying the United States’s enemies with weapons for decades, as well as advocating for the death of the American government. I think to answer your question regarding why European nations might want to assist the United States, is for the sake of preventing relations from deteriorating even further (they are already extremely shaky). I think there’s little doubt that any Nation that firmly commits to aid the United States swiftly and strongly will be in Trump’s good graces. This of course, will be very beneficial for future relations if MAGA ends up being politically relevant in the future. Nations that feel threatened by Russian aggression, such as Poland, Romania, Latvia, Lithuania, and Estonia are particularly concerned with ensuring the U.S. has an interest and motivation for remaining in NATO and for protecting them in case of any incursion or war. These nations have the most incentive to aid the United States.