United States

The U.S. is also difficult to invade. The United States are the current world power. When it comes to air and water support, no country comes close to the United States. 

Even its enemies are scared of fighting with the USA. The United States has had victories in many wars since the First World War.

Also, the country is protected by two oceans – the Atlantic and the Pacific. This gives the country a natural defence. 

Additionally, it also has many military bases both at home and abroad. That’s the reason they have military bases in war-tone areas. Of course, they are also regarded as the leaders of the world summits.