Question

What incident occurred that caused an international uproar and for britain to join the war?

Answer 1

Germany's instrusion of neutral Belgium was not going to be ignored.

Answer 2

Britain entered World War I in 1914 to honor a treaty that promised to defend Belgium and to fulfill an alliance with France and Russia. The Germans attacked Liege, Belgium, in the first battle of World War I, and then marched on to attack France.

I'm assuming by "war" you mean World War I