The U.S. Is playing a small but complicated role because they aren't fighting but they are supplying rebels but we don't know if they are terrorists also we are bringing 10k refugees but we don't want to start a war with Russia
The United Nations doesn't step in because if any of the powerful countries like Russia,China,U.S.,France,and Great Britain step in and say no the United Nations don't step in and Russia and China won't let the United Nations step in because they are friends with Syria and will get more money