Snatching victory from defeat...

Waterloo

Who turned unexpectedly against, or to support, Napoleon, out of gratitude?

I guess you could say that Masada was a victory snatched from defeat, on the Roman's part. In 72 BC, Lucius Flavius Silva marched against Masada with his Roman legion X Fretensis (meaning 'tenth legion of the sea strait - Rome's elite legionnaires) and laid siege to the almost impregnable fortress.
After failed attempts to breach the wall, they built a 'circumvallation wall' and then a rampart against the western face of the plateau, using thousands of tons of stones and beaten earth.
The rampart was complete in the spring of 73BC, after approximately two to three months of siege, allowing the Romans to finally breach the wall of the fortress with a battering ram on April 16th. When they entered the fortress, however, the Romans discovered that its 936 Jewish inhabitants had set all the buildings -save the food storerooms- ablaze and committed mass suicide rather than face certain capture(and defeat/public humiliation) by their enemies.
To the Romans, this task seemed almost impossible. Though they outnumbered the inhabitants, the job of actually getting to the fortress walls seemed very hard indeed. Silva's men had almost given up after numerous failed attempts at merely making it up the steep cliffs. The revolutionary wall/siege 'hill' was what tipped the balance. All but 2 women and 5 children killed each other (intentionally, rather than committing suicide which was discouraged by Jewish Law).
This was an incredible turn of events in favour of the Romans, are fairly legitimate for "snatching victory from defeat'.