Final answer:
General Sir William Howe decided to target Philadelphia and George Washington instead of supporting General John Burgoyne's campaign to capture Albany, contributing to a crucial strategic misstep for the British.
Step-by-step explanation:
It was General Sir William Howe who decided to go after George Washington in Philadelphia rather than aiding General John Burgoyne in capturing Albany. This strategic choice was made despite an existing plan, approved by British leadership and Prime Minister Lord North, which aimed to isolate New England by having three armies converge at Albany. Unfortunately for the British, Howe's decision to capture Philadelphia, although temporarily successful, led to a fateful division of forces, contributing to the eventual failure of Burgoyne's northern campaign.