Aside from everyone nuking each other into mutually assured obsolescence, the World War 3 worst-case scenario for Americans would be if the conflict reached the American mainland. An invasion. A massive assault.
The ideas seem unlikely, considering that the last time there was a full-on war on the American mainland was during the American Civil War. Previous to that, there were some foreign invasions but our land has been protected from battles against other countries almost two centuries.
But is it really so far fetched?
There is a great deal of debate about whether it could happen. I can't predict the future, but I can analyze the possible threats and make suggestions for preparing for such an event.
In all of this uncertainty, one thing remains sure: it would be an entirely different ballgame if the battle came to us.
Why have we been safe in previous World Wars?
There are many theories about why the United States has been safe during previous conflicts. Here are a few reasons that foreign armies haven't invaded.
The Second Amendment: Americans are armed to the teeth. Most other countries on the planet do not have an armed citizenry. The fact that almost half of the country is armed and that there are more than 357 million civilian firearms (and that is just the number of guns that are registered – unregistered estimates boost the number as high as 600 million) means that a foreign army on the ground would face major resistance. However, in certain anti-gun strongholds like Chicago, New York City, and Southern California, the risk of resistance would decrease accordingly. As well, an unfortunate downturn in gun ownership also weakens us.
Trained and organized forces: We already have an organized military, from our armed forces to the National Guard. Add to this our police forces, sheriff's departments, and our veterans, and there are many trained people on our soil at all times. Many states have citizen militias that train regularly as well.