The UK has officially begun the Brexit process. Here’s what happens now.

It’s official: The United Kingdom is leaving the European Union.

On Wednesday, UK Prime Minister Theresa May submitted notice to the president of the European Council, Donald Tusk, that her country would be departing his union under Article 50 of the EU’s Lisbon Treaty. This marks the formal beginning of EU and UK negotiations over what the British and EU relationship will look like after Britain leaves. If the negotiations don’t produce any agreement, then the UK will simply be kicked out of all EU agreements — including, most notably, the EU’s internal free trade and free migration agreements.

That outcome, often referred to as a “hard Brexit,” would be a disaster. Many of the 3.5 million EU citizens living in the UK would no longer be sure if they could legally remain in Britain. The British economy would be at serious risk: About 44 percent of British exports go to the EU, so new UK-EU trade barriers could well cause a new British recession.

So what’s about to happen is two years of intense negotiations to try to avoid that outcome. The goal will be to develop a compromise deal on key controversies, like migration, that would allow for a “soft Brexit” — one in which the UK retains some access to the EU’s single market.

But the EU and the UK are starting very far apart, with diametrically opposed goals on the fundamental issues. And because all 27 remaining EU countries and the European Parliament need to agree to a final deal, there are all sorts of reasons it could get vetoed. A ton of unpredictable events, ranging from the French far-right candidate winning a presidential election to Donald Trump offering Britain a trade deal, could upend the negotiations entirely between now and 2018.

The point, in short, is that Britain has just armed the clock on a ticking time bomb. And nobody knows if it will be defused.

Now that Prime Minister May has officially given notice to Tusk, the next step is to begin negotiations about the negotiations. In about a month, the UK and EU will formally sit down to come to terms on how the negotiations will work.

“Most of the formal stuff that will be agreed upon in the big meetings has already been penciled in,” Tim Oliver, an expert on the EU at the London School of Economics, tells me. “But that doesn’t mean they can overcome the sticking points about what issues to consider, what order to consider them, and so forth.”

Ultimately, Oliver believes, “nothing substantive” will be agreed upon until after the French presidential election in April and the German parliamentary election in late September. That’s because the French and Germans are, by far, the two most important EU member states. Without a firm sense of who their leaders will be in the coming years, it will be impossible to know what terms the EU might agree to.

After these elections, the real negotiating will begin. The EU and UK will need to come to agreements on the final terms of the UK’s departure from the EU. Some key issues include:

Once those issues are substantively agreed to, the UK and EU will need to work out a transition plan. If the final deal limits UK access to European markets, for example, they will need to work out how many years it will be before European tariffs on British goods go up. If EU migration to the UK is going to be restricted, they’ll need to figure out when the UK is allowed to start applying those restrictions and how strict their migration limits can be in the near term.

The transition issue is important because the UK and the EU are deeply linked. UK products, for example, are currently built according to EU regulations. If the UK isn’t going to be governed by those rules anymore, UK companies will need some time to adjust — to redesign their products for a more heavily non-European market.

If the UK and the EU Commission, the EU’s executive branch, manage to come to an agreement about all of this, things still aren’t done. Every EU member state then needs to agree to this final arrangement, as does a majority of the European Parliament.

Theoretically, the two-year provision of Article 50 creates an incentive for this to get negotiated quickly, as a hard Brexit benefits no one.

“It would certainly be a worse economic problem for the UK than the EU, but it would be a very bad economic problem for the EU,” Jeremy Shapiro, research director at the European Council on Foreign Relations, says.

But at the same time, it’s very plausible that a deal won’t be made within that time. The EU is too large and bureaucratic, and the number of veto points too large, for something this complex to be resolved quickly.

“One of the most difficult things in these negotiations is the fact that even if the EU wanted now to do a good deal with the UK, it almost certainly couldn’t do it within those two years,” Oliver says. “The EU isn’t equipped, really, to negotiate a speedy exit.”

If two years pass without a deal being reached, the EU and Britain may jointly decide to temporarily extend all existing EU agreements with Britain — making it an EU country in all but name — until final status negotiations are finished. That means we could be at the beginning of a very, very long negotiating process.

The best way to think about the negotiations is a kind of push-pull over three main issues: migration, trade, and sovereignty.

The Schengen Agreement requires that every EU country agree to let citizens of other EU countries live and work within its borders. This led to a large influx of EU migrants to the UK, especially after poorer post-communist countries joined the EU in 2004.

Backlash to this immigration was a key driving force behind the Brexit referendum’s success; 62 percent of “Leave” voters, according to a poll of 12,000 Britons, said that immigration was “a force for ill” in the UK. Prime Minister May vowed, in a January 2017 speech outlining her Brexit policy, to “get control of the number of people coming to Britain from the EU.”

Another key reason for Brexit was frustration with EU control over British laws. Leave voters saw EU institutions, like the European Court of Justice, as limiting British control over vital concerns like industry regulations. May vowed in her Brexit speech that “we will take back control of our laws and bring an end to the jurisdiction of the European Court of Justice in Britain.”

Yet at the same time, the UK wants to maintain its access to the EU common market, arguably the key benefit to remaining in the EU. It wants the economic privileges of remaining in the EU without the politically controversial international laws and permissive migration rules.

It’s a sort of “have our cake and eat it too” demand, and the EU is having none of it.

“The EU has been very strongly signaling that they’re just not going to give access to the common market without the free movement of labor,” Shapiro says. “That would really damage the very central notion of the common market, and it would also give incentives for other countries to ask for all kinds of opt-outs.”

Which means the EU arguably has an incentive to make Brexit as painful a process as possible.

Public opinion has swung against the EU since the Great Recession and subsequent eurozone economic crisis. Right-wing populist movements — all of which are deeply hostile to the EU — are rising around the continent. The EU needs to explain why these populists are wrong — to show that if a country leaves the EU, it will suffer. If it lets Britain leave with a sweetheart deal, then far-right politicians in countries like Greece and France could argue that they could depart the EU with ease. If more countries do leave, the whole union could theoretically start to unravel.

So a deal that’s really good for the UK economy and popular with UK voters is objectively bad for the future of the European Union. That means the two sides are starting very far apart.

Ultimately, the only way to avoid a painful, hard Brexit will be for both sides to compromise somewhat — to come to mutually acceptable agreements on migration, sovereignty, and trade. This isn’t terribly complex as a matter of policy: One could imagine some kind of compromise where Britain retains favorable trade rights with the EU and agrees to give EU immigrants special rights in Britain. But politically, such an agreement would involve painful compromises for both sides, meaning that even successful negotiations will likely be fraught and angry for a time.

“I can see a real crunch where both sides reach an impasse, and they have to back off — take a significant break,” Oliver says. “It’s not so much complex [policy]; it’s political willingness by both sides to agree to certain ideas.”

To add to the fundamental difficulty of the negotiations, there are lots of events outside of the Brexit process that could change the dynamics of entire negotiation.

The first, and most obvious, is the upcoming French election. It’s possible, the polling data suggests, that far-right populist Marine Le Pen could win the presidency. Not likely — most polls have her losing to her top rival, Emmanuel Macron, by a fairly large margin — but still possible.

Le Pen is staunchly anti-EU; her campaign is promising to take France off the euro within six months of victory. If she wins, then one of the most powerful countries in the EU will have become fundamentally hostile to the institution, shifting the internal balance of power in the EU in a way that’s likely to strengthen Britain’s bargaining position.

“Everything ... completely goes out the window if Marine Le Pen is elected,” Shapiro says. “It completely changes the incentives of what’s, right now, the most hard-line [anti] Brexit state — and obviously one of the most powerful actors in the EU.”

There’s also the not-small matter of the United States. Donald Trump has repeatedly endorsed the UK vote to leave, even referring to himself as “Mr. Brexit” during the campaign. If he were to offer the UK a favorable free trade deal with the US upon its departure from the EU, that could lessen the economic pain of Brexit considerably — and strengthen Britain’s negotiating hand by making a hard Brexit more palatable for the UK.

On the EU’s side, there’s the not-insignificant matter of the United Kingdom’s viability as a country. The UK is technically made up of four separate “countries” — England, Scotland, Wales, and Northern Ireland — that currently function more like US states. Yet Scotland and Northern Ireland both voted to stay in the EU, and political leaders in both countries have threatened to leave the UK as a result of Brexit.

Of the two, Scotland is the more pressing problem. The most popular party there is the pro-independence Scottish National Party; just one day before the official Article 50 announcement, Scotland’s parliament passed a vote in favor of holding (another) referendum on Scottish independence. This puts pressure on May to come to some kind of deal that preserves a stronger relationship with the EU.

“It’s the issue of Scottish independence, and another referendum, that genuinely does scare Theresa May — because she does not want to be the prime minister that loses Scotland and sees the breakup of the United Kingdom,” Oliver says.

Finally, you add on top of that all the different incentives of the EU’s 27 member states. While many are likely to agree to whatever agreement the UK and the European Commission arrive at, some might not. Spain, for instance, is using the vote to stake a claim to Gibraltar — a peninsula on Spain’s southern coast long controlled by Britain. Poland and Hungary are run by deeply euroskeptical parties who might push for a deal that weakens the EU as an organization.

The point, then, is that this is an immensely complicated negotiation with a ton of moving parts. Not only are the two main parties involved very far apart on the core points of disagreement, but there are lots of other actors who have the ability to throw a monkey wrench into the proceedings.

That means the odds of the worst possible outcome — a hard Brexit — are higher than you might think. Not inevitable, or even necessarily likely, as nobody really wants it. Rather, it’s just that the negotiations are so involved that any number of unforeseeable things could end up dooming them.

“Some people thought, actually, Brexit’s not really gonna happen; she [May] wants a soft Brexit,” Oliver says. “But this has been in the cards for a while. People are slowly waking up to this.”

CORRECTION: This article originally identified the UK as part of the EU’s Schengen Agreement. Free movement between the UK and (most) EU countries is instead provided for under the European Economic Area.