The Sicilian Expedition
When a nation is engaged in a long war that has settled into a stalemate, it often attempts to break the deadlock by opening a new front, attempting to catch its opponent off guard. It almost always ends in disaster. Both Hitler and Napoleon invaded Russia because even though they had forced Britain off the Continent, they couldn't invade the island. And the Sicilian Expedition was Athens' downfall during the Peleponnesian War.
The Second Gulf War was America's Barbarossa.