Ironically, none of the countries in WWI were actually obliged to enter into the war by virtue of their treaties.
The Triple Alliance didn't kick in unless one of the parties was "without direct provocation on their part, [...] attacked or [...] engaged in a war with two or more Great Powers nonsignatory to the present treaty" (or Italy attacked by France alone). But France and Britain declared war after Germany did, so the "two or more" doesn't kick in. Not to mention that the "direct provocation" is an ample loophole for anyone wanting to wriggle out of the treaty (as Italy, in fact did).
The Triple Entente wasn't a single treaty. The Franco-Russian Alliance was the strongest, requiring France and Russia to attack Germany should Germany attack one of them or merely mobilize in response to any of the Triple Alliance mobilizing. But again, this is a defensive trigger, and Russia mobilized before Germany did [1], so the clause didn't kick in. The Anglo-French Entente was not backed by any military alliance treaty, nor was the Anglo-Russian Entente. The Treaty of London merely guaranteed Belgian neutrality, and did not oblige anyone to enter into war against someone who violated Belgian neutrality.
For extra irony, in WWII, Britain and France went to war against Germany in defense of Poland, with whom they did not have a binding defensive treaty, but not in defense of Czechoslovakia, with whom France did.
[1] We know now that Germany actually decided its full mobilization before Russia did, but held back on announcing it just long enough to have the Russian mobilization announce first. In the event, France actually did not mobilize until after Germany mobilized against it, the German military having convinced Kaiser Wilhelm that mobilizing against only Russia was impossible.
Ironically, none of the countries in WWI were actually obliged to enter into the war by virtue of their treaties.
The Triple Alliance didn't kick in unless one of the parties was "without direct provocation on their part, [...] attacked or [...] engaged in a war with two or more Great Powers nonsignatory to the present treaty" (or Italy attacked by France alone). But France and Britain declared war after Germany did, so the "two or more" doesn't kick in. Not to mention that the "direct provocation" is an ample loophole for anyone wanting to wriggle out of the treaty (as Italy, in fact did).
The Triple Entente wasn't a single treaty. The Franco-Russian Alliance was the strongest, requiring France and Russia to attack Germany should Germany attack one of them or merely mobilize in response to any of the Triple Alliance mobilizing. But again, this is a defensive trigger, and Russia mobilized before Germany did [1], so the clause didn't kick in. The Anglo-French Entente was not backed by any military alliance treaty, nor was the Anglo-Russian Entente. The Treaty of London merely guaranteed Belgian neutrality, and did not oblige anyone to enter into war against someone who violated Belgian neutrality.
For extra irony, in WWII, Britain and France went to war against Germany in defense of Poland, with whom they did not have a binding defensive treaty, but not in defense of Czechoslovakia, with whom France did.
[1] We know now that Germany actually decided its full mobilization before Russia did, but held back on announcing it just long enough to have the Russian mobilization announce first. In the event, France actually did not mobilize until after Germany mobilized against it, the German military having convinced Kaiser Wilhelm that mobilizing against only Russia was impossible.