When Krishna established dharma, why did it last for such a short time?

From: Jugal

Krishna says that he descends to establish dharma, what does he mean by that? My question is because if we observe then the pandavas didn’t rule for that long after the war and soon kaliyuga began, so was the dharma that was established was just for few years? If it is that way then why such a big war? Please give the proper understanding.