It's a lesson the United Kingdom, unused to negotiating as the weaker party, is starting to absorb.
Britain entered the Brexit negotiations thinking it held five strong cards: the financial depth of the City of London; its large contributions to the EU budget; its "security surplus"; its relationship with Angela Merkel; and its size compared to Ireland.
But these -- valuable while the UK was a member of the EU -- were no use after its decision to leave.
London approached its negotiation of a "deep and special partnership"
as though it was another opt-out from the European project. It assumed that British cooperation was valuable and could be traded for market access.
It imagined the referendum result strengthened rather than weakened its hand. Britain now officially disliked the EU, and therefore that Britain needed to demand a higher price from its European partners for its continued cooperation.
But in Brussels, Paris and Berlin, this looked like more British extortion. With Brexit, the time had come finally, to stop paying Britgeld. (Danegeld was the tribute Anglo-Saxon kingdoms once paid to their Viking occupiers).
Insisting at one point that "no deal is better than a bad deal," Britain threatened to walk out of negotiations. The EU -- judging that the UK was negotiating with a gun pointed at its own head -- called London's bluff.
Brussels divided the negotiations into two phases. It wanted the UK to make "sufficient progress" dealing with the collateral damage that Brexit caused for EU citizens rights, the EU budget, and the Northern Irish border before moving on to discuss the future EU-UK relationship.
On Monday morning, as the Irish Taoiseach (Prime Minister) drove into work in his singlet, it seemed the British had caved in on all three: to pay its outstanding liabilities, accept something indistinguishable from ECJ jurisdiction to govern the rights of EU citizens living in the UK after Brexit, and agree to "regulatory alignment" for Northern Ireland with the EU and avoid a physical border.
Were the UK the unitary state still referred to in France as L'Angleterre, that deal -- however uncomfortable for May -- would now be done.
But the UK's political system has started to resemble a federation. She chose to ignore its constituent nations when plotting her path to a hard Brexit, which would take Britain outside of the EU's Single Market and Customs Union.
Now they are rising up against it. Her Toryism may dominate the English countryside, but it comes a distant second in Scotland, Wales and the capital. (Northern Ireland has, for its own historical reasons, a separate political spectrum organized on sectarian lines).
Nicola Sturgeon, First Minister of Scotland, and Sadiq Khan, mayor of London, have both asked: If Northern Ireland can have "regulatory alignment" with the Single Market and Customs Union, why can't we?
(Even Ruth Davidson, the Conservative leader in Scotland, said in a statement on Twitter:
"if regulatory alignment in a number of specific areas is the requirement for a frictionless border, then the Prime Minister should conclude this must be on a UK-wide basis.")
Everyone, that is, except the Democratic Unionist Party, the Northern Irish grouping on whose parliamentary support Theresa May's government depends.
The DUP can count on the support of just 36% of Northern Ireland's votes. But because of Britain's electoral system -- and the fact that for their own ideological reasons, Sinn Fein choose not to take their seats there -- the DUP are the only party representing Northern Ireland in London.
And whereas Dublin wants to avoid an economic border between Northern Ireland and the Republic, the DUP opposes one between Northern Ireland and Britain.
Now May is stuck. If "sufficient progress" can't be reached by the next European Council meeting in two weeks, businesses will begin to activate plans for a disorderly Brexit. But if it is reached, the Democratic Unionists could bring her government down and prompt a general election that her Conservative Party could use.
May could probably face down the DUP with the support of the Scottish Nationalists: but their price would be Scotland getting the same deal as offered to Northern Ireland. London's demands for autonomy, on the other hand, can probably be resisted for the moment.
But with Scotland and Northern Ireland staying aligned to the EU's economic rules, "united" would be the last word that could be applied to this kingdom. Better to call it the Former United Kingdom of England and Wales. I'll leave you to work out the acronym for yourself.