What is the Battle of Armageddon?

The word Armageddon is the Hebrew word [Har]megiddon which means mountain or plain of Megiddo. Megiddo is located in northern Israel in the valley of Jezreel. In ancient Biblical times battles were fought there.

If you look closely at the map, you will see the half tribe of Manasseh to the left and Megiddo in the northern part of it. 

In the 15th Century B.C. Egyptian Pharaoh Thutmose III fought against a group of Megiddo and Kadesh rulers. There are some that believe this is the same Pharaoh Moses and Aaron confronted when they demanded that Pharaoh let the Hebrews go free in the book of Exodus. 

During the time of Joshua's and his army defeat of many kings, it is mentioned that the tribe of Manasseh (one of the 12 tribes of Israel) received the region of Megiddo (Josh 12:21).

This is also the area where King Josiah of Judah is killed in 609 B.C. when battling against Pharaoh  Neco (2 Kings 23:29 - 30, 2 Chr 35:20 - 35). 

The word Megiddo is also mentioned in I Kings 9:15, "And this is the reason for the labor force which  King Solomon raised: to build the house of the Lord, his own house, the Millo, the wall of Jerusalem, Hazor, Megiddo, and Gezer."

That region has been the place of several battles in history and will be the place of a final battle - in essence World War III. Revelation 16:16 says that all nations will gather themselves to fight at this place - the plains of Megiddo. Or more popularly called Armageddon.

The armies of the earth are defeated by Jesus Christ

Revelation 19:11-21, "Then I saw heaven opened, and behold a white horse! The one sitting on it is called Faithful and True, and in righteousness he judges and makes war. His eyes are like a flame of fire, and on his head are many diadems, and he has a name written that no one knows but himself. He is clothed in a robe dipped in blood, and the name by which he is called is the Word of God. And the armies of heaven, arrayed in fine linen, white and pure, were following him on white horses. From his mouth comes a sharp sword with which to strike the nations, and he will rule with a rod of iron. He will tread the wine press of the fury of his wrath of God Almighty. On his robe and on his thigh he has a name written, King of kings, and Lord of lords. 

Then I saw an angel standing in the sun, and with a loud voice he called to the birds that fly directly overhead, "Come, gather for the great supper of God, to eat the flesh of kings, the flesh of captains, the flesh of mighty men, the flesh of horses and their riders, and the flesh of all men, both free and slave, both small and great." And I saw the beast and the kings of the earth with their armies gathered to make war against him who was sitting on the horse and against his army. And the beast was captured, and with it the false prophet who in its presence had done the signs by which he deceived those who had received the mark of the beast and those who worshiped his image. These two were thrown alive (italics mine) into the lake of fire that burns with sulfur. And the rest were slain by the sword that came from the mouth of him who was sitting on the horse, and all the birds were gorged with their flesh. 

The Apostle John is miraculously transported to the future, to the end of time as a matter of fact. He sees numerous visions which he records in his revelations, hence the name of the book - Revelation [(no "S" :)]. 

The above mentioned verses describes in incredible detail a final battle which most Biblical scholars believe will take place in the plains of Meggido. This battle comprises the armies of heaven led by Jesus Christ himself against the armies of the earth led by its leaders. The result will end in a gruesome blood bath where the earth armies will lose so that horse and rider along with the nobility of the earth and with all the ignoble that side with them will be devoured by ravenous birds.

After this victory by Christ, he will usher in the Millennium which is a thousand year period of global peace (Rev 20:2-7).