Mark Drakeford is right. The threat to the UK’s union must be treated as a major question of our time. Wales’s Labour first minister says the union has failed to keep pace with devolution and is fracturing before his eyes. From Cardiff, Drakeford sees little sign that the UK government is remotely bothered. Boris Johnson’s “aggressive unilateralism” was making things worse, the first minister said on Monday, the taste for “slogans, buildings and flag-flying” boosting separatism. But it doesn’t have to be this way. Differences should be a source of strength, and could be, if Britain had the devolutionary reset that it needs. Drakeford makes this case from a position of renewed strength, having seen off both the anti-devolutionists and the separatists in May’s assembly elections. Historically, the worm in the bud of UK devolution is that it has been piecemeal. Northern Ireland got home rule within the union in 1921. Scotland had its own Whitehall department from 1885; devolution followed only in 1999. The Welsh Office came in 1965; devolution, with fewer powers than Scotland, again in 1999. There has never been an English Office, nor an English parliament; devolution within England, once a land of local government, has been haphazard. The 1999 settlement had an idealistic side, but it was fundamentally driven by political self-interest. Labour re-embraced devolution in the 1990s in part because it shared the national feeling of Scotland and Wales (though not England). Mainly, though, it did so to block a nationalist surge if a Labour Scotland again faced a Conservative UK government acting with the aggression that Margaret Thatcher unleashed there in the 1980s. Wales was treated as a second-order issue by comparison, while England was ignored entirely. The problem for the 1999 settlement is that the central threat scenario never played out. Instead, in 2007, Labour collapsed in Scotland anyway. As a result, for 14 years and counting, under both Labour and the Tories, the SNP has positioned itself as Scotland’s champion in a union it attempts tirelessly to subvert and shatter – not, as Labour would have positioned itself, as Scotland’s champion in a devolved union that it wants to make work. One irony is that while Labour’s underlying assumption has never come to pass in Scotland, it has regularly played out in Wales. Labour has been in power at Cardiff Bay throughout the current period of Conservative UK government. Wales’s experience is thus a much more authentic test of what the 1999 settlement was envisaged as achieving. It also means that Drakeford should be listened to as a witness of unparalleled authority on its weaknesses. So you had better believe him when he says there is an urgent need for a new compromise. But that can no longer be another set of piecemeal changes – an extra tax power for Scotland here, police devolution for Wales there. To be stable, the reset has to be an agreed compromise for the union as a whole, a joint UK project based on shared sovereignty located in four different legislatures. This goes way beyond the self-interest of Labour or any other party. The political problem, very obviously, is that there is currently no other government in the UK that wants this. The Conservatives are dismissive, even if individual Tories are not. The SNP is actively hostile. The Irish nationalist part of Northern Ireland’s power-sharing agreement has no interest in strengthening the union by reforming it. In spite of all, though, this needs to be tackled now, on a cross-party and non-party basis, to have the slightest hope of curbing the UK’s continuing descent into separatism. The task is to enable reasonable people with differing mandates to work for the shared good within institutions that respect one another, and which share sovereignty. It will have to be federalist in ethos, if not strictly federal in structure. Subsidiarity and consent must be at its core. Above all, it must be centripetal not centrifugal, as the current system now is. That is a daunting challenge, especially given where we are now in both Westminster and Holyrood, and it is at odds with these islands’ history in many ways. It leaves Drakeford with little alternative but to be its chief UK voice, not only its Welsh one. It cannot, though, as the Welsh government document Reforming Our Union inevitably does, leave England to one side. The corrosive England-Britain elision that remains at the heart of the UK parliamentary settlement – not to mention UK football coverage – will not easily be undone. But it cannot remain untouched. Martin Kettle is a Guardian columnist