Final answer:
The change in diplomatic relations between Germany and England was mainly due to the development of alliances and the perception of encirclement. The Entente Cordiale exposed England to a possible war with Germany. The major weakness of the Schlieffen Plan was the assumption of a swift victory over France before engaging Russia.
Step-by-step explanation:
In my view, the most important reason for the change in diplomatic relations between Germany and England from good in 1890 to the brink of war in 1914 was the development of alliances and the perception of encirclement. Germany felt threatened by the alliances formed between Britain, France, and Russia, which were intended to neutralize German territorial ambitions.
The Entente Cordiale, a series of agreements between Britain and France in 1904, exposed England to a possible war with Germany. These agreements sought to resolve colonial disputes between the two nations and resulted in a closer relationship.
The major weakness of the Schlieffen Plan, Germany's military strategy for a two-front war, was the assumption of a swift and decisive victory over France before turning to engage Russia. The plan relied on a rapid and overwhelming defeat of the French army, but it did not take into account the possibility of a more robust defense by the French or the potential for British intervention, which ultimately led to the plan's failure.