Why did the French and English hate each other for around a millenia, but as soon as WWI and WWII roll around, they are suddently best buds?

It's worth noting that the Anglo-French alliance during the world wars did not come out of nowhere - the French and British had worked together as early as the 1850s during the Crimean War. In any case, I'm sure others can summarise why the changing international situation led to new understandings between the French and British - my own take would be that first in an imperial sphere, then in European politics after the First World War, the French and British found themselves as status quo powers. That is, the way things were set up suited them, and they were willing to work together to preserve this state of affairs. But there is some interesting new research coming out of a project headed by Peter Jackson at the University of Glasgow that turns your question on its head somewhat. That is, does the weight of Anglo-French rivalry throughout history act to sabotage this new alliance, particularly on the British side? Do British statesmen ever quite get past a way of thinking that views France as their 'natural enemy'?

Jackson uses this starting point to address the question of why the wartime relationship between France and Britain collapsed so quickly after the end of the First World War, but points also to the rocky relationship between the countries in more recent decades, such as on the question of British integration into Europe. This approach borrows heavily from recent work in memory studies, positing that the past is inherently malleable, endlessly reconstructed in the present based on changing needs, contexts and expectations for the future. In particular, Jackson points to the importance of anxieties in shaping policy, depicting history as a reservoir from which answers might be found to solve future dilemmas. History, in this depiction, forms part of a decision making chain, where previous experience and future expectation are in a constant, often self-perpetuating cycle.

The Anglo-French relationship provides fertile ground to observe the ongoing influence of history in understanding the present. Particularly for a certain breed of traditionalist thinker, Anglo-French rivalry and animosity is very much a live trope. France is portrayed as culturally and strategically other – incomprehensible, fickle and untrustworthy, very much a historical enemy, not least in battles from Trafalgar to Agincourt that are constantly refought in popular representations and memory. France is Napoleon, Louis XIV, the constant continental power whose potential for dominance implicitly threatens British interests. This ignores, of course, that France and Britain have been at peace for two centuries, and have fought several major wars as close allies in that time. In the grand, sweeping view of history that appeals to certain minds, these are temporary exceptions to a longer rule.

While there is certainly reciprocity at play – French views of England and Englishness do occasionally revert to somewhat accurate stereotype – Jackson points to their particular salience in British official thinking after the First World War to explain the rapid divergence in perceived interest between the two victorious powers. While French policymakers, Clemenceau in particular, were forced by the experience of invasion and occupation to address hard questions about their future security and plot a new way forward, British policymakers reverted to earlier ways of thinking with astonishing rapidity. Key to understanding this divergence is each sides’ view of history. For France, this history centred on 1870, and the sudden loss of continental pre-eminence. German invasion was the existential threat of the future, as it had been twice before in living memory. This led to a two-pronged approach to security – the strengthening and support of new Eastern and Central European powers as a counterweight to Germany on the continent, and the forging of a trans-Atlantic democratic bloc. Britain, it was now clear, was essential to future French security.

For British policymakers, however, the view was different. Germany, shorn of its fleet, was no longer an imperial threat, but rather a valuable potential economic partner. Rather, France assumed its age-old role as Britain’s nearest imperial rival, the strongest continental power on land, sea and air. Moreover, the needs of imperial defence, and the increasingly independent security demands of the Dominions, mitigated against continental security commitments. The underlying motivations of French security demands were therefore treated with suspicion. Would enabling an ascendant France, able to dominate the continent, simply be inviting a new challenger to imperial hegemony?

As Jackson points out, this historical view of France as a British rival had little to do with the present. France was exhausted by war, and painfully aware that German industrial and demographic potential outstripped theirs. This was an imagined threat, and one that needs to be treated with some scepticism, given that the armed forces were forced to protect their budgets in the aftermath of demobilisation, and cast around for new threats to defend against. Yet it is still telling that France could so easily be cast in the role of strategic rival, and that they – not Japan or the United States – were the most convincing new rival.

Sources

This is an unusual one - I was fortunate enough to hear Peter Jackson speak on this topic, and slightly less fortunately was forced to write a summary of the talk for the research centre blog, which I borrowed from liberally to refresh my memory of Jackson's thesis here. I believe that publications on this subject are forthcoming, but I've not found anything I can link to directly.

Edit: I found a link to the original blog post, which includes a recording of the talk if anyone feels like listening to it.