Complex sentence: It was adopted (along with Irving Berlin's "White Christmas") by American troops in Europe in World War II as a symbol of the United States--in fact, Garland even performed the song for American troops as part of a 1943 command performance.
Split into simpler sentences: It was adopted (along with Irving Berlin's "White Christmas") by American troops in Europe in World War II as a symbol of the United States. Garland herself performed the song for some American troops as part of a 1943 command performance.