WordNet sense 1 (a serviceman who has seen considerable active service):

veteran soldier

WordNet sense 2 (a person who has served in the armed forces):

vet, ex-serviceman

WordNet sense 3 (an experienced person who has been through many battles; someone who has given long service):

old-timer, warhorse, oldtimer, stager, old hand, old stager

WordNet sense 4 (rendered competent through trial and experience):

seasoned

WordNet sense 5 (having experience; having knowledge or skill from observation or participation):

experient