Rome’s first triumvirate was a power grab by Caesar, Crassus, and Pompey who sought to take the power of the Senate and share it among themselves. Crassus, the weakest of the three in political instincts, played an important role by siphoning off enough power to prevent a calamity between Caesar and Pompey. But, after his sudden death in 53 B.C, the six year old agreement became unstable as only Caesar and Pompey were left to fight each other for control of the Republic.
The second triumvirate, on the other hand, was sanctioned by the Senate as a legitimate source of consular power, because the elders had become too weak to resist anyone who would use military power to threaten them. The end point this time was the triumph of Octavian and the foundation of the imperial state.