As fate would have it, U2, Ireland's young, anthemic rock quartet, was booked into the same hotel here as the Army football team the day of the Army-Navy game. The whole Hershey Hotel was obsessed with the game, and in the hotel bar two scruffy band members watched it on a color TV, as puzzled as they were fascinated by the ritualized rivalry. "It's unbelievable," muttered the lead singer, known simply as Bono. "Depth charges versus artillery."

Far different is the Europa Hotel, where the band stays when it travels north from Dublin to Belfast. Caught in the middle of Northern Ireland's violent Protestant-Catholic factionalism, the Europa has been bombed 28 times. "You have to go through an army checkpoint to get in," Bono said. "When another attack happens, the staff goes, 'Oh, not again. Another day, another bombing.' "

After two atmospheric albums about coming of age, U2 turned outward last year and addressed the "Troubles" head on. The "War" album, with its stirring anthems about Northern Ireland, Solidarity and nuclear war, lifted U2 from new-wave cult status to the leadership of rock 'n' roll's most potent movement: the "positive-rock" or neo-folk-rock of Big Country, the Alarm, R.E.M., the Red Rockers, the Swimming Pool Q's, Simple Minds, True West, Aztec Camera and the Waterboys.

When U2 opened its current U.S. tour outside Philadelphia at the Tower Theatre in Upper Darby Saturday (the band plays the sold-out Constitution Hall in Washington tonight), the evening's highlight was "Sunday Bloody Sunday," inspired by the 1972 shooting in Derry of 13 unarmed Catholic protesters by British soldiers. Drummer Larry Mullen, his sandy hair cropped in a punk crewcut, banged out a noisy, rattling march beat. The guitarist, known only as The Edge, his high cheekbones and receding hairline giving him a long, gaunt look, added buzzing notes that sounded as much like a police siren in heavy traffic as a rock 'n' roll riff.

Bono, in black from his leather pants to his string tie held in place by a gold crucifix, sang, "Broken bottles under children's feet/ bodies strewn across a dead-end street." Then his big voice ripped free from its frustration and cried out, "How long must we sing this song? Tonight we can be as one." On the screen behind him flashed the photograph from the cover of "War": a young, tousle-haired Irish boy with big, brooding eyes that seem to ask the same question.

"Before we ever played 'Sunday Bloody Sunday,' " recounted Bono, 24, in the hotel bar, "we told the people of Belfast if they didn't like it, we'd never play it again. That was the moment. It was all or nothing at that point. We lived in Dublin, 100 miles up the road from Belfast, where the bombs are actually going off. So we felt the Belfast audience had the right to say if the song was appropriate or not.

"It was a mixed audience of Catholic and Protestant kids, and they exploded -- in the most positive sense of the word. Music can do what the politicians can't do. It can unite people, even if it's only for an hour and a half."

"It's one of those odd situations, but it's not unusual to have mixed audiences in Northern Ireland," added The Edge, 23. "The lead singer of Stiff Little Fingers once said that the solution to Northern Ireland's problems was 10,000 punk bands. The audience just lays aside any religious or political partisanship when they enter an auditorium -- they just come to hear the band. In fact, all the bands who had played Northern Ireland have said what an incredible audience they are. They need entertaining so much that when they actually get it, it's a tremendous relief."

How much of this unity carries beyond the night of the show? Bono, his black hair spilling out and down his back from a tall, crowned hat, paused thoughtfully for a moment, the football game glowing silently behind him.

"That's the million-dollar question, isn't it? Can rock 'n' roll do anything other than unite people for an hour and a half? I don't think it has to do anything other than that, but I would want it to. From the correspondence we get and from the people we meet on the street, I'm convinced that our music has become a sound track for change, certainly in our own country."

U2 was formed in 1976, when its four members all attended the same Dublin high school. They took their inspiration from the '60s idealism of Bob Dylan and Neil Young, from the avant-garde experimentalism of New York's Velvet Underground, Patti Smith and Television, and from the adrenaline energy of London's punk explosion.

"We took the love of the '60s and the hatred of the '70s," Bono said, "and somehow married them together into something new, which is definitely aggressive. You have to have both -- with one you're too uptight, with the other too laid back. You have to be a militant pacifist, an aggressive peacemaker. We're hardly flowers-in-the-hair people, but we don't have safety pins in our ears either.

"We're derided as dreamers all the time. But I think people ought to be dreaming more than they are in 1984. It's a nightmare people are living in the industrial wastelands of northern England and Wales, and I'm sure it's the same here in the Midwest. If rock 'n' roll can excite people into dreaming and believing that those dreams can come true, then it has accomplished something. Whether you call that naivete' or stupidity, for us it's a reality. We made it our reality. We were four people who formed a band before we knew how to play."

Eventually they learned, and they moved from garages to small pubs. By 1979 they were stars in Ireland. In 1980 they released their first British single, "11 O'Clock Tick Tock," and first album, "Boy," which made many year-end 10-best lists. The 1981 "October" album was a silver disc in Britain.

As one measure of the band's influence at home, Irish Prime Minister Garret FitzGerald this year invited Bono to join a committee on Ireland's youth unemployment. As another measure, Bob Dylan invited Bono to join him for an encore version of "Blowin' in the Wind" at an outdoor festival in Dublin before 40,000.

U2 shares not only a similar optimism and anthemic quality with bands such as Big Country, the Alarm and Simple Minds, but friendships, too. "I think we share a belief in music," The Edge ventured. "That it can be more than a chocolate bar that you unwrap, consume and then throw in the bin, that it has some staying power and cultural potency. It needn't be totally bland and unchallenging.

"A lot of bands have lost that idealism, I think. Maybe it's a product of coming from a big city like London or New York. When we were starting out, that geographic distance really helped us hang on to that romantic, idealistic edge. The whole punk thing in London had lost its original ground roots of enthusiasm and had metamorphosed into a commercial fashion industry. Because we were separate, we were able to miss that cynicism and just catch the energy."

The new U2 album, "The Unforgettable Fire," is named after an exhibition of paintings by survivors of Hiroshima and Nagasaki that the band is trying to bring to Dublin. Produced by avant-gardist Brian Eno, the album has been attacked by American critics for replacing the sharp impact of "War" with a diffuse murkiness.

"That makes sense," The Edge acknowledged, "because 'War' was a very American album, very monochromatic with well-defined songs. Whereas this record for me is more representative of the band's European, experimental side, which is a bit more out of focus, more ambient. I enjoy music that doesn't necessarily spell everything out, where there's room to interpret the lyrics for your own situation, where the music has less buzz and more textures."

The new single, "Pride (In the Name of Love)," is nevertheless a rousing anthem worthy of "War." It was the band's first top-five single in Britain, and it is one of two songs on the new album about Martin Luther King Jr.

"We started to say to ourselves," Bono explained, "how do you deal with a volatile situation like you have in Northern Ireland or Poland? And you look to a man like King, who, in a situation in the southern states of America that was just as volatile, managed to draw some positive life out of it, who believed in his cause enough to die for it instead of kill for it. This applies to our situation in Ireland.

"There is a cartoon picture, a black-and-white world view of Ireland. Our ambition is to get it out of the black-and-white and into the gray, where it belongs. It's very gray. I am the son of a Protestant mother and a Catholic father, so I grew up in the reality that all this religious conflict was utter bull. I never had anything rammed down my throat, and if anyone tried, I threw it up as quickly as I could."

There is a definite streak of Christian mysticism running through U2's songs, but the group is reluctant to discuss it "because everywhere we turn," says The Edge, "this spiritual industry in rags confronts us. Especially in America and in Northern Ireland, you see the total hypocrisy and contradiction of religion.

"Yet there's definitely something there. It's a step beyond that fallen Catholic thing you see in Martin Scorsese and elsewhere in the arts. It's not a frustrated thing, it's a positive awareness. It comes from growing up in Ireland, which is a very spiritual country -- it's steeped in mysticism and tradition. All I can say is that in understanding our music, you gain an insight into our spiritual beliefs. The opposite is not true. You can't understand our music by trying to figure out our beliefs."

U2 is perched on the brink of superstardom. A large throng milled outside the Tower Theatre, unable to get into the sold-out show. Inside, the young audience stood on seats and crowded the aisles. As a slide of King flashed behind him, Bono punctuated "Pride" with clenched-fist salutes that were returned by the swarm of white teen-agers near the front of the stage.

For the second encore, the band played "40," an answer to "Sunday Bloody Sunday" that promised, "I will sing a new song." Even after U2 had left the stage, the swaying fans stood at their seats and continued the anthemic refrain, over and over: "How long to sing this song?"