It’s hard to think of a country that has less to fear than the United States. Two vast oceans reduces vulnerability to attack, except by countries with sophisticated long-range ballistic missiles (5 out of 206 nations). It shares long borders with two nations that that count as close allies and trading partners.
Historically, the U.S. has only faced an invasion once, by the British during the War of 1812. (There have been other minor incursions, by Mexico during the 19th century and the Japanese occupation of two remote islands in the Aleutian chain during World War II. The Pearl Harbor attack was an air raid, not an invasion.)
Objectively, the USA has little to worry about beside terrorism — and that’s a job for domestic police and intelligence agencies, not the military. Yet 54% of discretionary federal spending goes to the Pentagon. A 10% cut — $60 billion a year — would buy universal pre-school or allow half of America’s four-year college and university students to have free tuition. But Trump seeks to increase military spending by $54 billion — roughly 10% — per year.
The U.S. accounts for less than 5% of the world’s population. We account for 37% of military spending worldwide, equal to the next seven countries, China, Saudi Arabia, Russia, the UK, India, France, Japan, combined. (And the U.S. sells a lot of hardware to most of those countries.)
Russia spends roughly a tenth as much on defense as the U.S. And they have a lot more (and twice as much territory) to defend against: NATO/American missiles to their west in Europe, a southern border full of radical Islamists in unstable countries like Kyrgyzstan and Uzbekistan, Afghanistan a stone’s throw away, historical regional superpower rival China next door.