On second thought, if you're referring to the First Worldwar here, that's also not very logical. Even assuming there wasn't in place the intricate coalition system that made possible the war, Britian could hardly have stood by watching Germany overrun Belgium - whose neutrality was guaranteed by both Western allies - and France. Already the violation of Belgian neutrality was a casus belli, let alone (the possible collapse) of France.
The violation of Belgian neutrality was not mentioned in the British ultimatum to Germany, the declaration of war, or Grey's famous speech to the Commons. This was done for two very good reasons.
The first was that Britain was not legally permitted to intervene militarily on Belgium's behalf by the terms of the guarantee; four-power consultations were required, and they had to be acting in concert. So any British attempt to justify their war by citing 'poor little Belgium' would have been quite illegal.
The second was that Belgium
didn't figure into British calculations. It was not the fact that people were violating Belgian neutrality that induced the Cabinet to fight. Britain certainly would not have attacked
France if the French had been the first to set foot on Belgian soil. British policy was aimed against Germany, in concert with France, and
Germany alone. That lunatic Grey had got it into his head that the German Empire's industrial and military power was a 'violation of the balance of power', which as we all know is never a justification for anything because nobody actually defines what a 'balance' is. He had permitted that distasteful ass Henry Wilson, of the Imperial General Staff, to conduct secret staff talks with the French for years
while at the same time baldly lying to the Cabinet about the talks and British commitments to France. Britain was already committed to France in the minds of Grey and, more understandably, of the governments of the French Republic; the only reason Grey prevaricated at all during the July Crisis was because he and Asquith knew they couldn't count on the Cabinet to abide by his secret-treaty backroom warmongering. And the only reason Asquith permitted Grey to issue the ultimatum is the revelation from Bonar Law that the Conservatives would support war (if it came to that; the Liberals were looking awfully iffy about the whole thing), based on the words of the double-agent Henry Wilson (!), who'd been passing information on the Curragh mutineers to the Conservatives, and who'd convinced Bonar Law that a civil war in Ireland was inevitable unless some foreign distraction was found to unite Britain and Ireland behind King and Country.
I am not making this up. It was
really that bad. The original story of moral crusade to save the neutral small states of Europe fits in better with the wartime propaganda of the nasty Hun into a very pretty package that Barbara Tuchman could sell you in an admittedly well-written book, or that Fritz Fischer could use to contrast to the supposed German premeditated plot to invade the rest of Europe. Both total nonsense. The Entente Powers had a unified war plan for fighting Germany and Austria-Hungary (which was set on a tripwire - if any one of the Entente states went to war, the others would've mobilized within two to three weeks...German staffs knew this, yet the supposed 'Schlieffen Plan' had a six-week time table?
); the German and Habsburg general staffs were barely on speaking terms, and constantly misled each other into the bargain. Talk of a 'balance of power' was ridiculous, because Germany and Austria-Hungary had fleets and armies that were totally dwarfed by those of France and Russia alone, never mind the United Kingdom. And Russia's were still growing
very rapidly...with British and French credit, of course.
The Belgians were the official reason (and the one that guaranteed widespread support for the war), but the real reason was to defend the balance of power by protecting France.
Oh, they weren't the official reason either. Just the newspaper-reason. It was easy to capitalize on that stuff after stories of atrocities started to filter through (and half of these were made up too, though to be fair there's a helluva lot of truth to the whole horrific tale). People get pissed about stuff like Louvain; they don't get pissed about the head duck and a hungry ostrich getting themselves shot.