Try this version.
“He must have known what he signed up for .... and still he was willing to serve this great Nation that he loved.”
Beat me to it, my thoughts exactly!
“He must have known what he signed up for .... and still he was willing to serve this great Nation that he loved.”
Or another version...
“He knew what he signed up for, and he knew if he did not do it, someone not as good as him would have gone. And more people would die. That is what a hero is.”