At one time the Celts nearly ruled the ancient world then suddenly they just withdrew stopped fighting and trying to expand their territory why?
They were not a united nation. Their expansion into Europe was not the result of a national policy but merely the result of local tribes attacking and defeating local neighbors and it was very gradual. When they made contact with peoples, such as the Greeks and Romans, who could muster greater resources than the local Celtic tribe, they could go no further. In addition, they were always fighting one another. They did not really see themselves as one people set against outside, non-Celtic people. One Celtic tribe would be just as likely to fight with a neighboring Celtic tribe as with a non-Celtic one. Michael Montagne
The Celts were too war-like and very hard to control. They basically self-destructed.
"The Celtic tradition of preferring indiviual combat over regimented organisation made the Celts look for alternative ways to resist. Guerilla tactics, resistance and even outright retreat, when outnumbered, gave Caesar the impression the Celts were "cowards"; but nothing could be further from the truth. In fact, Celtic leaders had trouble keeping their warriors from attacking Romans strong points head-on."
There is an excellent article, Vercingetorix: Gallic Warrior against Rome Contents, that gives the reader some insight on the Celts. The link to this article can be found in the Related Links to the right.
In addition, I think you are confusing the Celts with the Mongols, who did conquer perhaps the largest land empire in world history, and did suddenly recede from Europe just before conquering it.
If you are referring to the true Celts, then the answer is that there were no one "Celts".
Rather, it was a large, very diverse group which shared a common derived language. As answer #1 points out above, the "Celts" were really many different tribes of folks, and not some single nationality.
