/u/elos_ could probably weigh in here as well, but in short, the past century leading up to WWI had seen a number of examples of Franco-British cooperation, such as the Crimean War (much of the Great Game in general) and the Suez Canal. The zenith of Franco-British Antagonism, the Fashoda Crisis during the Mahdist War in 1898 resulted in the Franco-British Entente Cordiale. This was solidified and led to military cooperation in light of the Moroccan Crises, and German sabre-rattling and pretensions at continental hegemony. The confronting of a common enemy, Germany, was essentially the key proximate cause for France and Britain becoming allies.
I can't comment too deeply on other aspects of the development of the Anglo-French alliance in the early twentieth century, but I know a little bit about the naval scope of the whole thing.
By the late nineteenth century, there was a broad consensus amongst British strategists that the Royal Navy no longer enjoyed the primacy that it established at the end of the Napoleonic Wars, and that it could not solely guarantee the security of the Empire. Consequently, London would need to rely more on other powers to assist the Royal Navy in protecting its global interests. In the Mediterranean, British naval power had been waning over the years, and while the Royal Navy was still the most powerful in the region, that advantage was gradually eroded by the resurgence of other naval powers, particularly France and Russia, and the emergence of new powers such as Italy.
| Country | Battleships in 1883 | Battleships in 1897 (plus those in building) |
|---|---|---|
| Britain | 38 | 62 |
| France | 19 | 36 |
| Germany | 11 | 12 |
| Russia | 3 | 18 |
| Italy | 7 | 12 |
| USA | 0 | 11 |
| Japan | 0 | 7 |
As you can see from these numbers, by the end of the 1800s, the Royal Navy's lead was quickly deteriorating, not just in the Mediterranean, but in other theatres like East Asia and the North Atlantic. To be sure, it still enjoyed numerical and technological superiority over any other single power, but an alliance between even a small handful of the great powers would significantly diminish or even wipe out that advantage.
And this gap was only going to narrow further. The growth of the United States as a major industrial power at the end of the nineteenth century challenged Britain's long-standing dominance in the North Atlantic, and Japan's dogged focus on establishing itself as a first-rate indigenous naval power in East Asia threatened poorly-defended European territories in the Asia-Pacific. Above all, the meteoric rise of Germany and its Imperial Navy imperilled the British Isles themselves. Great Britain, having been the first nation to industrialise, had enjoyed an enormous lead over her rivals, and having taken advantage of their relative weakness, shrewdly exploited that lead to establish herself as the world's first true global power. But, as industrialisation spread to other nations, that position had more or less evaporated by the end of the nineteenth century. The British could no longer rely on their country's immense industrial capacity to simply out-produce their opponents as these powers approached and, in the case of the United States, surpassed, British industrial output and sophistication. This fundamental change in the balance of the global economy made it much more difficult for the Royal Navy to maintain its unilateral dominance in every theatre, especially the Mediterranean with its glut of great powers on its periphery, without some form of materiel assistance from the regional actors there.
This new strategic reality put the British in an increasingly precarious global position. The re-emergence of France as a major naval power in the Mediterranean meant that, should the French decide to militarily assert themselves in the region as they had attempted in 1898, the Royal Navy's ability to check them was not necessarily guaranteed, and, should the French and Russians ally, the Royal Navy's Mediterranean Fleet would find itself greatly outnumbered. The strategic significance of the Mediterranean Sea to the British Empire at the time cannot be understated: had a power or consortium of powers blocked British access to the sea, by extension, they would have barred access to the Suez Canal, which would have further isolated British India and put the entire Empire in the Eastern Hemisphere at risk.
This overarching trend forced the British to admit that France was no longer a second-rate naval power that could be discounted, but a potential threat to British overseas interests, and that it had to alter its long-term naval doctrine accordingly. This set the stage for the British to come to an agreement with the French in 1912 in which the French would agree to help secure British interests in the Mediterranean and the British, in exchange, would assist with protecting France's Atlantic coast in the event of a war with Germany. Great Britain had now thrown herself back into the chaotic world of continental great power politics, a game from which it had largely disengaged itself for the past eighty years.
I don't really want to write about this any more, but I hope that I have helped anyone reading this understand what was going on at the time or that I haven't made any glaring errors, but most importantly, I hope I haven't broken any cardinal r/askhistorians rules.
Source: Kennedy, P., The rise and fall of British naval mastery, London, Penguin Group, 2001, pp. 205-237.