Pleas identify a single country that has the desire and realistic ability to attack our "Homeland" ?
Who says we have to be imperialistic war makers who waste our blood and treasure "protecting" the rest of the world, and invading, bombing, and bribing the countries that don't want our "protection".
The Koreans, Indonesians, Japanese, Chinese, Philippines, Australians, Saudi Arabians, Israelis, Germans, Egyptians, French, Italians, and all the rest of the world,
think we are idiots for bankrupting ourselves to protect their countries for them,
or wasting our money invading, occupying and bombing them,
so they can use their money competing for our jobs, and providing health care, education, and housing for their citizens.
As a Canadian friend told me once, "our country spends its money on education and health care - and we let you spend your money protecting us - which of us is smarter ?"
To answer your question who has the desire to attack our country? Russia, China, N. Korea, Iran, and any number of other countries that hate our guts.
To answer your second question who has the ability? For sure Russia and China. N. Korea is very near or we wouldn't be putting up a missile defense systems in Alaska. Iran could be nearer than you think as Iran and N. Korea share missile technology.
It would take only one nuclear bomb exploded 150 miles into our upper atmosphere to cause a huge electromagnetic pulse to destroy our country. Just Google EMP and you will be terrified. If you aren't then something is wrong with you.
As far as the U.S. defending all the countries that it does, is wrong. These countries should try to take care of the defense cost of there own countries. Why the U.S. at the very least doesn't charge them for defense protection is beyond me. Why should we pay for their defenses?
One other aspect that I failed to mention. Just follow the money trail to defense contractors. There is a lot of money to be made off that. But there are a lot of jobs that go along with that. Maybe as many as a million or more workers might be laid off and be jobless if they were stopped. How much of the defense industry keeps our countries economy moving? I am betting a lot. It is a double edged sword.