If the U.S. were to enter such a war with the world against Russia, we would be somewhat of a minority in the war. Here are some reasons.
1. Statistics show only ~250,000 soldiers in the military (you have to be kidding, I would have expected more from a country populated with 300 million people!)
2. The government gets so scared of soldier loss so they refuse to send in more.
3. Referring to #1- pure fear of death by citizens which is the reason they won't join the military. Our military is supposedly well trained, but it seems the numbers are down so much because people aren't committed to the hard work of training and being away from home to defend our country and help others.