A military invasion is a large-scale operation that involves sending armed forces into a foreign land to take control of the territory. The operation can be conducted on land, by sea or by air. Invasions can also be a part of a larger conflict between a nation and a non-state actor, such as terrorists or pirates.
Military invasion requires a massive amount of resources, and the advancing force must outnumber the defenders by a significant margin. This is why many operations that involve aerial deployments of soldiers or the use of naval and amphibious forces are referred to as invasions, even though they may not be a true invasion in the strict sense of the word.
The United States is a vast country, and invading it from any direction would require a very large military force to overcome the natural obstacles (forests, swamps and mountains) as well as the logistical problems (i.e. bringing enough troops and supplies to the battlefield). This could quickly deplete available resources, making the operation very costly.
In addition, the defenders have to deal with continuing opposition from civilian and paramilitary resistance movements. This can be difficult to eradicate, but pacification of the region through the use of media propaganda and other means can reduce this desire for continued resistance, which is often motivated by anger and revenge. Ultimately, this pacification reduces the likelihood of future violence and can make the occupation more successful.