England & France have not always been besties. We all know it. However, I was always curious what changed in the late 1800s/early 1900s where over hundreds of years of prior conflict became a strong alliance in 2 world wars, as well as in founding the EU, being part of NATO/UN, etc. Point is, I've recently heard in a lecture that Germany was responsible for these two becoming allies in the first place. My teacher told us that Germany's naval power threatened British supremacy in the seas, and France was a surefire ally to any anti-German nation after the Franco-Prussian War, so the two teamed up. However, is there more to this? Did England not see France as a threat at all? Was France not jealous of British dominance abroad, or did they see an alliance with such a powerful country as helpful in maintaining their colonies in North Africa, for example? Did any other European countries (Italy, Austria-Hungary, Ottoman Empire, & Russia come to mind, for their various roles in WWI) play into this forming of an alliance? Because as a French-American living in the States, it seems to me like the Modern Era relationship between the French & British is a real special one overall.
Germany is certainly a large slice of the pie yes, but there is a lot of water under the bridge by the time the first world war comes around.
By the turn of the 19th/20th centuries, Britain was long established as the dominant world power, most notably due to it huge and unrivalled navy. Britain's pre-eminence was proven over France in the 19th century and this was established.
By the time we cross into the 20th century, both Britain and France's imperial expansionism centred on Africa. You may have heard the term "scramble for Africa". Well that's what was happening, Britain and France were scrambling to take (for want of a better word) ownership of as much of Africa as possible before the other could.
Wars - especially those far from home - are exceptionally expensive, and so the scramble for Africa lead to Britain and France cooperating and coordinating over who could take what, rather than fight costly wars over the issue. This worked out for France as it was well established by this time that Britain was the super power of the world, and it worked out for Britain who's foreign policy at the time was of maintaining the status quo in Europe. In other words they wanted to focus elsewhere, not fighting war after war with France.
With the backdrop of all this, Germany is getting itself together as a powerful and industrialised country. And once Bismark is out the way (thanks to Kaiser Wilhelm II), its own foreign policy became very aggressive - toward the more established powers, i.e. France & Britain, but also Russia.
One of the things Wilhelm did was create a large navy. He thought that by having a massive navy he would be able to challenge Britain for pre-eminence, without ever having to actually use the navy. This was obviously very provocative and this kind of thing drove Britain and France further into each others arms.
The lead up into the war, the arms race (i.e. the navy expansions), created a level of tension that necessitated formal alliances for self protection & preservation. This lead to Britain and France formalising an alliance, along with Russia (the Triple Entente) to defend each other against the Triple Alliance which was posturing loudly (Germany, Austria, Italy).
The event of World War I and its aftermath then essentially solidified this friendship between Britain and France and set the tone for the new norm. They won the war together, worked together to create a new post war Europe and then did it all again 20 years later for World War II.
EDIT: I forgot to explicitly mention the Franco Prussian wars but since you mentioned it in your original question it should be addressed.
France and Germany obviously are quite close to each other geographically, in fact they share a massive border. This issue is around the unification of Germany in 1871. Prussia was the leading germanic state up until that point and under Bismark, thanks to things like industrialisation and good rail networks, Germany united under Bismark and Wilhelm I which worried France (a new and powerful neighbour) which lead to the Franco Prussian war. I'm not an expert on this, perhaps someone else can add to this, but essentially, after this, France and Germany were not best friends and France was worried about future wars with Germany - it had already happened, they needed allies if it was going to happen again!