When Napoleon I was readying his forces to retake Paris in 1814, he had addressed his men on the offensive and they were in support. Yet when he went to take the plan to his commanders, Ney (with the support of the other marshals) replied that they would do no such thing and surrender to the Coalition, famously stating "Sire, the army will obey its leaders." in regards to the future actions of the Grande Armee. While any offensive would have likely been a hopeless cause, what caused this final defiance now, and not later nor earlier?
In 1814, the Allies were pushing into France; Napoleon was fighting some of his best battles since the 1807 campaign. He was pushing back the horde as best as he could but it wasn't enough, the Allies were pushing in and pouring more troops than Napoleon could handle. By that time, the French army was in shambles, not from defeat but from exhaustion. The army being no more than a forty thousand men, couldn't hold back the combined might of the allies, combine that with the change of attitude of the Allies which turned into almost absolutely frantic as Napoleon consistently defeated greater armies but did nothing to change the mood of the Allies.
So, from here, the Marshals come to Napoleon. Paris was taken a few days before and Napoleon was frantically trying to find ways to continue the war and reclaim Paris. However, Ney was able to pulled several Marshals to beg Napoleon to sue for peace. They were exhausted. For nine years, they marched with Napoleon from Paris to Moscow but were becoming exhausted, and most of them fought longer than that in with previous Revolutionary experience. They were given privilege and prosperity, and they wanted to keep it as most of them had pulled themselves up from "commoner" level to that of high aristocracy. They wanted to live a comfortable life and hope that they could keep it if Napoleon would surrender.
Further, it was honestly hopeless. I've mentioned the field army Napoleon had, slowly it would go down to about ten thousand towards the end (and still pulling victories) but it couldn't stand against the combined Allied army that took Paris (well over a hundred fifty thousand soldiers). Napoleon here realized that it was truly hopeless, he couldn't win. The final act of defiance has more to do with the timing, Napoleon could have won in 1813 if not for a few mistakes and turns of allies against him, 1812 should have been a victory but the Russians were able to win time by giving Napoleon the space to push into Russia, previous campaigns were clear victories so there was never dissent. Here was the end, and Ney helped Napoleon see it.
Often people do compare Napoleon to Hitler, pushing Europe to a decade of war and death and I've always fought the perception. Here, this comparison is the most contrasting. Napoleon, looking at the map of France, is willing to hear out his commanders and realizes that it can't be done; Hitler wanted everyone to fight to the death. People say that Napoleon was cold and ruthless but he understood the pain and suffering he was putting on himself and France, and in the end he put France above all.