Why was it unusual for Britain to participate in World War I?
Yes, it is. The Tri-Entente between Britain, France and Russia was formed in 1907 as a response to the Tripartite Alliance of Germany, Austria-Hungary and Italy in 1882.
Germany, Austria-Hungary and Italy. Italy wants to be with the rooster (France).
However, the Triple Pact was entirely an informal understanding in which no one party was committed.
Britain's entry into the war, at least publicly, was based on Germany's failure to guarantee the integrity of Belgium, which the British (as well as the German Confederation, etc.) promised in the Treaty of London (1839).
The London Conference of 1830 led to the Treaty. The horse is Prussian and the fox is English. The rabbit is French and the bear is Russian.
Britain is by no means a military power (as far as the army is concerned), it has only 250,000 regular troops (about 700,000 including territories and reserves), while the German army has 700,000 regular troops (including 3.8 million reserves).
This is what led the Kaiser to call Britain a "despicable little army", which the British unusually liked quite a bit. Since then, the original British regular army has been called "old and immortal" by everyone.
It was clear that this small army was not intended to be drawn into a full-blown European war, and therefore Britain's declaration of war on Germany on 4 August 1914 (one day later than France and three days later than Russia) was by no means an inevitable conclusion, but very unusual. Britain had avoided involvement in the previous Prussian-Danish, Austro-Prussian and Franco-Prussian wars, and there was no particular reason to be involved in the war.
In the end, despite being completely unprepared for a continental war, the British made the decision to support Belgium and France against Germany, fearing that France would suffer another major defeat like the Franco-Prussian War of 1870. The British feared that this would lead to Germany's unquestionable dominance in Europe, an outcome the British did not want to see.
Arguably, if Britain had not participated in the war, it would have ended in a few months and would not have become a global conflict, saving millions of lives.
