Why did the Second Crusade fail?

The failure of the Second Crusade is attributed to a combination of poor military execution, strategic miscalculations, and deep-seated internal divisions among the participants.12

Key factors in the failure included:

• Conflicting Interests: The Crusade was undermined by the competing agendas of the European monarchs, the local barons of Jerusalem, and the Byzantine Empire. These divisions prevented a unified strategy and led to mutual recriminations.12
• Poor Execution: The campaign against Damascus was poorly conceived and executed. After a failed siege, the Crusader army was forced to retreat, leaving the mission unfulfilled.12
• Military Losses: Both the French and German armies suffered heavy casualties during their arduous journeys to the East, particularly at the hands of the Seljuq Turks.12
• St. Bernard’s Idealism: St. Bernard of Clairvaux, who promoted the Crusade, was an idealist who failed to account for the "quarrelsome nature of politics, peoples, dynasties, and adventurers". He projected his own integrity onto the Crusaders, many of whom were driven by different, often conflicting, motives.3

Following the collapse, St. Bernard attributed the failure to the sinfulness of Europe, asserting that only through purification and prayer could victory be achieved.1