The US should lead the opposition to this act of war (yes, invading a sovereign nation and its territorial integrity is considered an act of war) with economic and diplomatic sanctions against Russia. Get everyone on record with who they support, including the UN, EU, and Nato to investigate any situation threatening international peace; recommend procedures for peaceful resolution of a dispute; call upon other member nations to completely or partially interrupt economic relations as well as sea, air, postal, and radio communications, or to sever diplomatic relations; and enforce its decisions.
Let Russia come across as the brutish invaders and reap the repercussions, they are the ones who have sent militias across the border. Basically, the US needs to lead the world into making it more expensive (politically or otherwise) to stay in Ukraine. France, the UK, and the US will stand together, but how China responds will also be key.
The point here, is that one nation can not invade another, and there are going to be those who do something about it-and those that dont. We are at a distinct disadvantage with a weak president who not only advertises his unwillingness to use force but also has squandered his political capital at home and abroad. An actual leader wouldn't even be in this mess=Obama's foreign policy has been appallingly bad.
