The Ambush on Thanos was a semi-successful planned attack organized by the Avengers and aimed at reversing effects of the Decimation by retrieving the Infinity Stones.
Unfortunately, Thanos had destroyed the Stones to avoid temptation, rendering this goal impossible and forcing the Avengers to settle for the hollow-at-best victory of killing the Mad Titan.
History and Ambush
After Thanos successfully destroyed half of the universe with the Infinity Stones by snapping his fingers with the Infinity Gauntlet at the end of the Infinity War, he transported himself to Titan II with the Space Stone.
Three weeks later following the Decimation, Thanos used the Infinity Stones for a second time to destroy themselves in order to avoid being tempted to use their power and knowing the Avengers would attempt to reverse the Decimation with them, though this ended up scarring him and the Infinity Gauntlet even further.
Two days later, while at his farm making a stew from his crops, Captain Marvel appeared and held him in a headlock while the rest of the Avengers showed up as part of an ambush, with Thor ultimately cutting off his left arm with the Stormbreaker.
However, to the team's horror, Thanos revealed what he has done in advance. The Avengers accused him of lying only for Nebula to reveal that Thanos never lies. Thanos thanked his daughter and admitted he shouldn't have been too cruel with her before an enraged Thor cut off his head in retaliation.