Playing Risk against all AI. In this game, the AI was fairly stupid, which usually made for an easy match. Unfortunately, this time the stupidity worked against me.
I had enabled the maximum # of opponents and decided to start in Austalia, for the early continent bonus. However, one of my opponents had the same idea and just would not die. While I was fooling around there, Barbarossa was consolidating his position in N. America. Rather than attack him, all of his neighbors decided to fight one another, weakening themselves and preparing the way for his eventual dominance. To make matters worse, my flank in Asia was constantly attacked by two opponents who didn't seem to have any intention of building a respectable empire. While my Asian neighbors fooled around with annoying but ultimately pointless attacks and my Australian rival steadfastly went tit for tat with me in battle losses, Barbarossa continued his inexorable conquest of the Americas, then Europe and finally Africa.
By the time I eliminated my rival in Australia (who had no cards when he died, BTW) Barbarossa had moved into position to eliminate the final two rivals and gain their rather substantial card surplus. The situation was grim. I controlled Australia, my opponent had every other continent but Asia. I had one card, he had 6 or 7. I thought about my strategy, considered where to attack and where to defend, and gradually came to the realization that I had to attack. Barbarossa had too many potential reinforcements, and I had no hope of holding on to any continent for the forseeable future. My only chance was to rob him of his continent bonuses and hope to stay alive long enough to get some card bonuses of my own. With those, I might be able to assert control over a decent continent while simultaneously denying my enemy.
Here is where my luck suddenly turned for the better. I had a small army in SE Asia, to which I added all 3 of my reinforcements. I struck westward, taking India and the Middle East without losing a soul. I then took Egypt and West Africa, denying his African bonus. I sent a small force north into Western Europe to eliminate his European bonus, then continued into Brazil, finally driving north into Central America. Throughout this miraculous drive I lost not a single soldier.
At this point the AI stupidity won the game for me. Rather than destroying my continent bonus in Australia and recapturing the Americas, Barbarossa spread his numerous reinforcements thin and counterattacked utterly unimportant territories. He took a few central Asian countries from me, then ended his turn. He didn't even reinforce N. America! The next turn I took Africa and South America, which stayed mine. The turn after that, with help from my cards and newly acquired continents, I reduced my opponent to a few meager territories in Siberia, where he was to meet his end. Never before or since have I made such a comeback.