The Greeks have a word for it - hubris.
Thucydides, the Greek historian, reminds us that in the 5th century BC the Athenian Greeks learned a lesson about having too much hubris - arrogance. The citizens went to the agora and voted to send a military expedition to what in those days was a distant Sicily. There the Syracusans were harassing a small nation-state that was something of an ally of Athens. On the day the expedition rowed away in 134 triremes - warships with three banks of oarsmen on each side - most of the populace of Athens came down to the harbor at Piraeus to cheer and send them off.
There was no reason to doubt victory. Syracuse was no more than a backward, uncultured nation-state on the outskirts of civilization. When the Athenians arrived, however, the military of Syracuse tried some new tactics that confounded the Athenian generals sufficiently that victory did not come quickly, as anticipated. Instead, the Athenians had to send home for reinforcements. After two years of war, the Athenian force in Sicily was so decimated that few managed to return home. This was only the beginning of problems for Athens, however. In another nine years the Athenians had lost their empire abroad and their democracy at home. The hubris that had carried them to Sicily had started them on the road to their downfall.
Is it not time for us to recognize that there was a good deal of hubris behind our decision to invade Iraq? It impelled Congress to pass a resolution in support of an attack, the president to decide to invade, and the American public to give wide support to his doing so. The initial fighting went well, but the enemy's tactics since have not been what we anticipated.