With the centenary of the end of World War I in November, there have been plenty of sombre occasions for European leaders of late. But perhaps none have been quite as sombre as the brief meeting to sign the withdrawal agreement detailing the UK’s departure from the European Union.

Unlike the war commemorations, during which the human cost of conflict could be counted and old enmities mused over, this meeting dealt with a very current issue, and one that is not going to get any easier in the foreseeable future.

Usually, when national leaders meet in Brussels, they express their community publicly with warm words, often taking a “family photo”. It’s perhaps not surprising that much of the usual furniture of such meetings was absent this time. In the minds of many in the EU, this was not a day for celebration or commemoration, but of business, and a sad one at that.

It is no secret that there has been widespread dismay and disappointment at the UK’s choice to leave the European Union, even if there has also been a clear acceptance of that path. Such conflicting emotions were greatly in evidence as the signing approached, most obviously with the Spanish government’s behaviour over Gibraltar.

Despite securing an explicit mention of the territory in the withdrawal agreement itself, after extensive bilateral negotiations with the UK, the Spanish took it upon themselves to use the final moments of the Article 50 negotiations to flag concerns about the wording of one provision. This could, potentially, have been used to automatically give Gibraltar the same status as the rest of the UK in the eventual trade deal that is hoped to follow Brexit. Now, instead, Spain must be consulted before Gibraltar’s status changes.

For all that, the Spanish had no intention of derailing this summit. But that did not stop them using the opportunity to lay down a marker on Gibraltar’s status for the next round of negotiations – on the future relationship – that will begin after March 29, 2019.

We might take this as an early introduction to the world that the UK is now entering: hard-nosed pressing of advantages to secure national priorities. The EU will be sad to see the British leave, but this will not mean that they’ll give them an easy ride in the future.

To put this in some context, it is worth remembering that the texts that have been signed off represent only a small fraction of Brexit – and probably the simpler parts at that.

The withdrawal agreement is fundamentally a backward-looking document, providing decisions on how to resolve the UK’s liabilities from ending membership: citizens’ rights, finances, the Irish dimension.

Yes, there is a political declaration on the future, but it is just that, a declaration. A key part of Theresa May’s domestic ratification battle will turn on whether that more aspirational document allows for the pursuit of other, non-Chequers-type deals, and so make it worthwhile accepting the clearing of the decks that the withdrawal agreement represents.

The EU summit underlined that trade off, with leader after leader making very clear that the text is now set and that renegotiation is not possible.

Partly that’s about stopping the British from thinking that they can come back to the negotiating table in the event that May is defeated in parliament over it. But it’s also partly a reminder that, as a non-member of the EU, the UK cannot draw on the kind of favours that allowed it to get the assorted opt-outs that so characterised its time in the organisation.

United front no more?

But there is another important message from the summit. Spain’s behaviour might be understandable in the context of future rounds of negotiations and of looming local elections, but it has also very clearly annoyed other member states.

That said, the Spanish were hardly the only ones to start laying out markers: France, the Netherlands and Denmark all raised issues around access to fisheries, and others have been drawing up their plans, too.

If the Article 50 process has been marked by an exceptionally high level of unity between member states, then the same will not be true of the future relationship negotiations. The former was presented as an existential threat to the core values of EU membership, but the latter will be like other, more mundane negotiations with third countries.

In that, the member states have extensive experience and habits: even though the Commission will still be the negotiator for them, they will expect and act as if their individual national interests must be satisfied. If nothing else, the need for unanimous approval of that future text will mean that every countries’ issues are fully taken into account.

Given that those states have very different views about what that future relationship should look like, we might well expect the kinds of problems that led to the Walloon parliament’s initial rejection of the free trade deal with Canada.

If the process has been hard and downbeat so far, then it’s not going to get any easier from now on.