ABC News said the crowd of about 5,000 was one of the largest the probable Republican nominee had addressed during the campaign, but it wasn't clear whether they had all shown up to hear Romney.
The event, sponsored by a veterans museum and the Vietnam Veterans of America, is an annual event that attracts thousands of people each year, ABC said, and organizers insisted the gathering was not political.
However, the crowd was friendly to Romney, ABC reported.
The former Republican governor of Massachusetts framed the upcoming election as a choice between a weak military and a strong America, saying Americans "have two courses we could follow."
"One is to follow the pathway of Europe," Romney said, "to shrink our military smaller and smaller to pay for our social needs … and hope for the best. But if we followed that course, there would be no one stand and protect us.
"The other," Romney said, "is to commit to preserve America as the strongest military in the world, second to none, with no comparable power anywhere in the world."