In the First World War, Britain was a part of the Triple Entente, along with France and Russia. Thus they became one of the German army's primary enemies from 1914 & 1918. The British army was soon thrown into France in 1914, with the British Expeditionary Force, that began the war with approximately six divisions at its disposal. But as the war expanded so too did the BEF, which would include millions of soldiers.
Of course, it was not entirely a 'British' army. Britain expanded their armies with imperial divisions from around the empire. The empire gave Britain considerably more troops for the war. Australians, Canadian, Indian and New Zealand forces provided additional reinforcements for the British army during the period. With those divisions the overall British forces were among the largest of the war.
In 1914, the BEF soon arrived in France & Belgium as the German advance pushed the French back. In Belgium they slowed the German advance, but their armies could not halt it. Only at the Battle of the Marne did the British and French divisions combine to bring the German advances to standstill. There the BEF and French army countered, which ensured a German withdrawal.
After Germany's withdrawal, both sides established trenches in France. That ensured the front was somewhat static up until 1918. Britain's army was sucked into a number of indecisive battles, with neither side making notable gains during the period. But alongside the French, they held the line.
Britain's army was among the first to deploy a large number of tanks during the war. Although the early tank models were very slow, tanks such as the Mark IV, deployed in 1917, had greater impact. By 1918 their tanks would have considerably more impact in France, as Rawlison threw in hundreds for the Battle of Amiens. At the Battle of Amiens Britain's Fourth Army made considerable advances with their tanks providing greater infantry support. Then the German troops surrendered in numbers.
Beyond Europe, Britain also had other notable fronts. Although ineffective during the Gallipoli Campaign in 1915, their armies later regrouped in the Middle East. In 1917, they made an effective advance which ensured the fall of Baghdad. In 1918, Britain and their Arab allies won a notable victory at the Battle of Megiddo. With their cavalry, aircraft and tanks the British surrounded Turkish positions during the battle. Then the Ottoman Empire crumbled as Damascus and Aleppo fell to Britain. Such victories were essential for the defeat of the Ottoman Empire.
The British army had won notable victories in the Middle East, and their millions of reinforcements were fairly essential in France as well. An estimated 8.9 millionBritish Empire troops were provided. Had the British not declared war with the German Empire, then the Germans might well have broken through in France, and occupied Paris.