Someone was asking about Medicare exclusions from coverage (eyeglasses: excluded; dental: excluded).
Another one is exclusion of Medicare payment for medical services required "as a result of war or act of war."
So it seems if we had a continental war here, and there were 2 million Medicare age people injured, they wouldn't have health insurance. Whether this type of exclusion might be found in commercial plans; I don't know.
SSA 1862(a)(5)
https://www.ssa.gov/OP_Home/ssact/title18/1862.htm
No payment may be made under Part A or Part B for items or services required [a5] as a result of war, or an act of war, occurring after the effective date of the individual’s current Medicare coverage.
