The Conservatives are not only struggling to agree on a Brexit vision for the future, they are losing their grip on the past. John Major tried in vain to jog memories on Wednesday, warning that the government’s disregard for economic and diplomatic reality had set the country on a trajectory towards calamity. To identify the danger was not “Project Fear”, he said, but “Project Know-Your-History”.

For this, the former prime minister was denounced by Tory MPs as an embittered hypocrite, propagandist and traitor. Politicians are not obliged to like their former leaders, but Conservatives might at least be expected to muster respect for the experience of previous generations. That is what conservatism is for.

But a scorched-earth ethos has captured the party. On the forced march from reality nothing of pro-European Conservative culture can be left standing. This is the spirit in which some Brexiters have taken to trashing the Good Friday agreement. By locking north and south of Ireland into mutually beneficial integration, the treaty impedes clinical severance of ties between the UK and the EU. So its success in underpinning peace for two decades must be rewritten as failure. In this view, history is not a record of events but an editable preface to whatever political expediency demands today.

An irony here is that recent Tory history and its lessons for Brexit are well understood on the other side of the Channel. European leaders have observed the gradual capture of a reasonable, mainstream party by the livid manias of its fringe.

One example: when David Cameron was competing for the Tory leadership in 2005, he needed a policy to shore up his Eurosceptic credentials. So he pledged to take the Conservatives out of the European People’s party (EPP), the main centre-right group in the European parliament. It was, in his eyes, a cost-free policy – potent as a symbolic rejection of European “federalism”; inconsequential in practice.

Merkel could see, as could any attentive witness, that no concession would ever placate the Eurosceptic ultras

Cameron honoured his commitment in 2009. Tory Eurosceptics were glad and few in Britain noticed or cared. One person who noticed and cared was Angela Merkel, whose Christian Democratic party is in the EPP. The German chancellor thought Cameron’s decision was potty, a wilful surrender of influence, a myopic flounce out of a forum for heavyweight leaders. The Tories instead cobbled together a club of angry nationalists and their influence in EU institutions declined accordingly. Merkel’s dismay was communicated privately to Cameron. She thought the decision marked him out as unserious. When he became prime minister the following year diplomatic efforts had to be made to persuade her that he wasn’t an idiot.

Over the following six years, Merkel watched Cameron being moved further and further away from the European mainstream. She could see, as could any attentive witness, that no concession would ever placate the Eurosceptic ultras and that, if continually indulged, they would not stop harrying their leader until they had destroyed him, which eventually they did.

Now the same Tory faction is harrying Theresa May. They were not satisfied with rebates or opt-outs when Britain was an EU member. They were not satisfied with Cameron’s renegotiation of that membership. And now they are not satisfied with any Brexit but the most extreme version. (If they get that, they will probably find cause to complain.) The hardline anti-EU ratchet keeps on turning and, just like her predecessor, May puts up no visible resistance. Merkel and her fellow European leaders recognise this process. There is an institutional memory in Brussels and, among officials, much fluency in the English language. European negotiators can read British newspapers.

The patterns of recent Tory history are familiar to May’s counterparts in the Brexit talks. They probably have a clearer-sighted understanding of them than she does. This matters because May’s pitch to the EU is that she can be trusted to uphold the values of the European project even while quitting its institutions. She offers “deep and special partnership” on matters of security and economic cooperation. She rejects the suggestion that Britain seeks to undercut its continental neighbours by dropping labour standards and environmental protections. She promises a post-Brexit partnership “based on high standards”. So May invites her EU counterparts to be generous and cooperative on the basis that she is an ally – aligned and equivalent in values if not exactly identical in regulations.

But even if the rest of the EU accept that May is sincere, they know she is weak. They know there is a section of the Tory party that is implacably hostile to the European project. That faction saw a regulatory bonfire as Brexit’s primary purpose. Some fantasised about a great unravelling of the union as a happy side-effect. That hostility is not a secret and the EU cannot ignore it. They have followed British history enough to know which side tends to win tugs of war between Tory leaders and Eurosceptic backbenchers. They knew where power lies in that party.

This is a problem for May beyond the technical challenge of negotiating post-Brexit trade. Assurances of good faith that the prime minister gives in Brussels are undermined by the record of ill-will in her party. The resilience and destructive capacity of that animus is well understood in Brussels. And if May needs reminding of the impact it has on Conservative leaders, she can always ask John Major.

• Rafael Behr is a Guardian columnist