Balancing the budget on the back of defense

I would balance the budget by cutting defense. Defense would be mainly defending our own borders, and since the US has never been invaded by a foreign force, defense should not cost that much. I estimate that defense could easily be cut by 80% based on the fact that the US spends roughly 10 times more than Russia or the UK or France, and about 7 times more than China. Even at 20% of current spending we would still be spending double what most countries spend and 43% more than China.