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Why did English fleets fail to relieve La Rochelle during the siege?

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Full-length portrait of Cardinal Richelieu by Philippe de Champaigne, showing him in red clerical robes holding a red book, standing beside a balustrade with a draped curtain behind. — Cardinal Richelieu As both statesman and churchman, Cardinal Richelieu was the acknowledged architect of France's greatness in the 17th century. In 1640, when this full-length portrait was painted (likely by Philippe de Champaigne), he was embroiled in a dispute with the pope and other religious controversies along with many other consequences of expanding French power across Europe. He would die two years later, facing conspiracies against his life and suffering ill health.

English fleets failed to relieve La Rochelle during the siege primarily due to the strategic military measures implemented by Cardinal Richelieu.1

To isolate the Huguenot stronghold, Richelieu oversaw the construction of extensive land fortifications and, crucially, a massive seawall that blockaded the channel leading to the city. This barrier effectively prevented English naval forces from accessing the port to provide supplies or support.12

Additionally, earlier English efforts led by the Duke of Buckingham were hampered by poor organization and military setbacks. Buckingham’s initial attempt to secure the nearby Ré Island failed, leaving his forces depleted by disease and combat before they were eventually forced to withdraw. Subsequent attempts by English fleets to bypass Richelieu’s blockade were also unsuccessful, leaving the city to succumb to famine and disease after a prolonged siege.13