The Battle of Armageddon is a great war which will occur sometime in the future when the armies of the Beast gather in the plains of northern Israel near the Hill of Megiddo, in order to fight against Jesus Christ. The Book of Revelation states:
"And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared. And I saw three unclean spirits like frogs come out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet. For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty . . . And he gathered them together into a place called in the Hebrew tongue Armageddon."
Revelation 16:12-14, 16 KJV
And also:
"And I saw the beast, and the kings of the earth, and their armies, gathered together to make war against him that sat on the horse, and against his army."
Revelation 19:19, KJV
Please also look at the topic
Armageddon, and consider reading the article
From Armageddon To The New Earth.