By December, morale was the lowest ever. "One [trumpeter] could not get to sleep at night until he'd had so much to drink that he would roll off his bed and sleep on the floor." One member wrecked Glenn's car, and other cars broke down. Probably the worst tragedy for the band was that of the drummer Maurice Purtill leaving to play in New York. In January of 1938, he decided that the band wasn't working, and they broke up.

March 1, 1939, Glenn was informed that his band was selected to play at the Glen Island Casino. Miller was surprised, because his selection was based on the performances at Paradise Restaurant, which he was not exactly proud of.  The Glen Island Casino gave the band its much-needed success, after quitting there in late 1939, the band started to break records. By 1940, the band was so successful that it could not fill all the dates offered. They had an extremely busy schedule, playing three radio shows a week, with rehearsals, five hours a night at the hotel, and four shows a day at the Paramount. In addition, that summer they recorded more than thirty songs.

Many of the songs they recorded together have interesting histories. For example, the group's theme song, "Moonlight Serenade" began in 1935 when Miller was working on a mathematical composition exercise. Mitchell Parish wrote the final lyrics, which became "Moonlight Serenade," and was recorded on the back side of the "Sunrise Serenade" record.

One of the most well remembered songs was "In the Mood." It was written by Joe Garland, but it lasted eight minutes, too long to record. Miller took the song and made his own arrangement to fit on one side of a record. Chattanooga Choo-Choo." was just written for a movie, but sold over one million copies, and Miller was presented with "a gold plated copy of his record, and this became the prototype of the Gold Record now awarded by the Record Industry Association of America for all million sellers.

After eight years of leading a band, as many bands do, Miller developed his own distinct sound. "The miller 'sound' was a clarinet lead, with a tenor sax - or saxes - playing the same melody one octave lower than the clarinet." Other trademarks of his band included the steady brass section, consisting of four trumpets and four trombones.

On September 27, 1942, Glenn Miller decided break up his band and go to serve his country in the war. Once in the Air Force, he wanted to assemble service bands to play marches and entertain troops, but found the process difficult because of strict military politics. Eventually he was assigned to the Army Air Forces Technical Training, where he did organize a band. Miller wanted the band to go overseas, where the action was, and the refusal with other military stress began to make him more irritable. Once, he ordered his band to shave their moustaches, so they would look like real soldiers. They did not like this, some of them had moustaches for many years, and it made the wind instrument players change their technique.

Glenn was very stubborn. He insisted on going overseas and applied for transfer using his full name, Alton Glenn Miller. Only then his application was accepted. In the spring of 1944, the army sent a group of about sixty, consisting of musicians, arrangers, producers, and announcers, to London, England. The band began to play familiar tunes for the soldiers who needed a taste of home. The band would have preferred to learn and play new songs, just for some variety in the identical gigs. But Glenn knew what the soldiers wanted to hear, and played it for them.

On December 15, 1944, Glenn Miller left in a plane for Paris, flying over the English Channel, and was never seen again. There are now three theories about how he died. The most accepted one states that the plane crashed due to poor weather conditions. Another one thinks that the plane was shot down by another plane of unknown identity flying higher than Miller's. The third is that he arrived in Paris, only to die of a heart attack, and the media made up the story of the plane crash to keep war morale up. We will never know for sure.

After his death, "Tex Beneke, his first chair sax-man and vocalist took over the band…Miller had finally became a musical success, yet his orchestra lasted a scant 4 ½ years." Miller was just forty years old at the time of his death, which ended all the grand plans he was making for his band after the war. As said by George Simon in The Big Bands, "Of all the outstanding popular dance bands, the one that evokes the most memories of how wonderfully romantic it was, the one whose music people most want to hear over and over, is the band of the late Glenn Miller."