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.
The factor that most directly resulted in Great Britain’s entering world war 1 was to defend Belgium and react against Germany’s violation of the 1839 treaty.
Explanation
German went to war with Belgium making Britain to be involved in the world war 1 because the 1839 Treaty of London that was signed by Prussia (which Germany was a successor state) required that the neutral state of Belgium should be respected by all members in the treaty. Britain involvement in the war reminded Germany of their obligations concern that treaty.
