Final answer:
Frederick arrived at Acre during the Third Crusade, with King Richard I of England being a major Christian leader in the campaign. They attempted to reclaim Jerusalem from Muslim forces but ultimately settled for a treaty allowing Christian pilgrim access.
Step-by-step explanation:
Frederick arrived at Acre during the Third Crusade, a pivotal campaign in response to the loss of Jerusalem to Muslim forces led by Saladin. King Richard I of England, also known as Richard the Lionheart, was one of the key Christian leaders of this crusade. However, the Third Crusade saw various leaders, including Richard, confront internal disputes and challenges, ultimately resulting in a treaty that left Jerusalem under Muslim control but allowed Christian pilgrims to visit freely. During this period, Richard I was likely involved in strategic military actions and negotiations, as he played a significant role in the Third Crusade.