VIERA, Fla. – After all the uncertainty, the flights to “home” games, and the dwindling support in their old city, the Washington Nationals finally played a game. And the debut – Washington hadn’t been represented on a major league diamond since 1971 – was a rousing success, in the stands and on the scoreboard.

Playing their first game since leaving Montreal, the Nationals opened spring training yesterday with a 5-3 victory over the Mets and new manager Willie Randolph.

After three years in limbo as the Montreal Expos, the team finally took to the field wearing home white uniforms with “Nationals” in red and gold across the chests and a curly white “W” on their hats reminiscent of the old Senators’ caps.

“It did feel good once I put it on,” second baseman Jose Vidro said. “And when I went out there, the people were cheering for us once the game started.”

The starters sprinted out on the field at 1:02 p.m., and a minute later came another first in Nationals history, albeit a tad more dubious: the debut of a team theme song, “We Are the Washington Nationals,” which sounded something like AC/DC. Blessid Union of Souls recorded the song and offered it to general manager Jim Bowden.

“I was wondering, ‘What’s going on?’ It was like head-banging, then it said ‘Nationals’ and it was pretty cool,” Brad Wilkerson said. “It’s pretty cool to have your own song. It was pretty upbeat. We didn’t have a song in Montreal.”

Washington starter Tony Armas Jr., who missed most of the past two seasons after major shoulder surgery, worked two hitless innings with two strikeouts. He threw 28 pitches and pronounced himself “pain-free.”

The three key new position players – Cristian Guzman, Vinny Castilla, and Jose Guillen – each got a hit. Vidro, who missed the final six weeks last season to have right knee surgery, singled and scored a run.

An announced sellout of 7,558 began arriving more than five hours before game time, hoping for a photo, autograph – or just a glimpse of a player. The line at the stadium gates was several dozen deep when they opened 1 1/2 hours before the first pitch.

Even the Mets were impressed by the turnout.

“It was pretty cool. It’s not often that the first game of spring training is nationally televised. I’m happy for those guys,” said New York starter Tom Glavine, who allowed two hits in two scoreless innings.

Chad Cordero struck out the side in the ninth, putting the Nationals’ first victory in the books. That was 2 hours, 33 minutes after the first pitch: a ball, outside, at 1:06 p.m.