Which factor most directly resulted in Great Britain entering world war l

The correct answer is: German troops invaded France through Belgium on August 4th, 1914, as part of the Schlieffen Plan.

Not only had Britain promised to defend Belgium under the Treaty of London of 1839, moreover German control of Belgium would have been seen as a serious threat to Britain.

Belgium's ports were close to the British coast and German control of Belgium would have been seen as a serious threat to Britain. In the end, Britain refused to ignore the events of 4 August 1914, when Germany attacked France through Belgium. Within hours, Britain declared war on Germany.