Think what it meant to sing this Christmas carol in 1942.
The United States and Britain were losing World War II. France was conquered. the next Germany offensive was expected to finish off Russia. No one would have been much doubted, if he predicted that no one but bloodthirsty dictators would come out of this war on top: a world given over to Hitler, Stalin, Mussolini, the Japanese warlords, Mao Tse-tung. Lights out.
This is the world into which Jesus Christ entered in the flesh. Because God intervenes in history, the lights that went out in 1942 were turned back on. Because Jesus Christ is King of Kings and Lord of Lords, we dare to sing to celebrate His birth, we dare to hope, we defy the powers of this fallen world.
Sing, sing, and sing louder!