When did German failure become virtually certain in WW2?

[Orser67]

While Stalingrad (and to a lesser extent, the concurrent Guadalcanal campaign) was a huge turning point, I think the Axis powers may have still had a chance of victory until later in 1943. During that year, the Allies more or less achieved victory in the Mediterranean theater, the Soviets won a major victory at the Battle of Kursk, and Churchill and FDR committed to invade France at the Tehran Conference.

But I think people sometimes ignore the possibility of some kind of negotiated outcome, which is what Germany was shooting for towards the end of the war. I think the Battle of the Bulge was the last chance the Germans had of anything short of unconditional surrender, though I could also see an argument for mid-1944 (Operation Bagration+D-Day).