It was the Crusades. The Crusades was a campaign to regain the holy land from the muslim invaders after they took it over. Some came to fight for their goal while others joined to see new lands while there were some riches. Many people were promised forgiveness of their sins if they joined the Crusade. Other effects were that it led to the growth of towns and cities as the nobility sold their lands to join the Crusades and people gained their freedom.