100 years ago today, the United Kingdom declared war on Germany, after Germany invaded Belgium. Britain had signed a treaty guaranteeing Belgium's neutrality (as had Germany) - Germany had decided they had to go through Belgium to invade France. Britain issued an ultimatum (lots of ultimatums were issued in the days leading up to the Great War) telling the Germans to get out of Belgium; Germany kept on coming. And so England joined the war.
There are lots of places where the war starts - July 28, when Austria attacked Serbia; July 31 when Russia mobilized; August 1 when Germany responded to that, and declared war - and of course the 4th. August 4 might make some claim to being where the Great War became a World War - without Great Britain, the war would have been mostly continental - maybe some colonial squabbling, but nothing like what would happen when England joined. It brought in Australia, New Zealand and Canada, for example, brought in India; it made the war more of a naval affair; it brought countries like Japan in. It also probably sealed Germany's fate - however long their odds of beating both France and Russia, they were considerably worse with England involved. Whatever ideas they might have had for their navy, for instance, became moot. It undoubtedly changed the shape of their submarine war. It made their position quite difficult.
Should England have fought the war? it is controversial - some claim that they lost far more than they could have ever gained by fighting. That is undoubtedly true - though the question is, exactly how long would they have been able to stay out? if they hadn't fought in 1914 - would Germany have provoked them to fight in 1915? what if Germany had won in 1914? would that have been a disaster? obviously not the disaster it would have been - and was - when Germany won in 1940 - but not likely to help England much.
The truth is - most arguments about WWI, and who should have done what, tend to founder on the course of the ear, and the aftermath. The war itself developed into a pointless bloodbath. The peace proved to be a disaster for all concerned. You keep looking for ways they could have done something different - not gotten into it, dug in and stayed dug in without trying to break the stalemate; Versailles is pretty easy to improve.... But it's kind of pointless. Everyone went in - well, it's the opposite of what Renoir said: everyone had bas reasons. Except, maybe, the British - there is something to be said for honoring your treaties and defending the national and territorial integrities of other countries. None of it did anyone any good - but the Germans (and Austrians, and in some ways, the Russians and even the French) are going to have their war come what may - and odds are everyone was going to get sucked into it. The US would get sucked into it eventually - I imagine the UK would haven even if they hadn't defended Belgium.