Not true. We lost wars of attrition in Vietnam and Afghanistan. The Russians also lost a war of attrition in Afghanistan. There are dozens of examples in history of a smaller country successfully defending its own territory against a larger country in a war of attrition. They win because they're more willing to pay the price of victory than is the larger country.
We didn’t lose the Vietnam war—we quit same with Afghanistan, same with Russia—they were not defeated—they just walked our—a Bit like England in the Revolutionary War.