What was the root cause of the change in Britain's WWI foreign policy? (Glorious isolation changed)

The traditional British foreign policy was to maintain continental parity in Europe and not to allow any one country on the continent to become excessively powerful. Later, the rapid rise of Germany and its overseas expansion seriously jeopardized the interests of Britain's overseas colonies, and the contradictions between Britain and Germany rose rapidly to become the main contradiction in Europe, and joining the Allied Powers was to stop Germany's overseas expansion and to maintain Britain's maritime supremacy.