Right. And why would Britain decide that Germany having territory on the Channel would be a cassus belli? Why side with Russia just a few decades after WW0 (the Crimean War)? Why did Russia attack Austria-Hungary?
Britain jumped in against Germany because Germany was warring with Britain’s ally, France. They knew that Belgium was just a stepping stone, one that they couldn’t allow.
Russia got involved in support of Serbia, Germany got involved because of Russia, France got involved because of Germany, The U.K. got involved because of France and Germany; the U.S. got involved because of the U.K. None of it was anyone’s business.
I think the U.S. got involved because the Japanese bombed the hell out of Pearl Harbor....
You need to read your history. Britain was unprepared for the war, but had to jump in to maintain the neutrality of the channel coast.