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(CNN) —  

You could be forgiven for thinking we’ve been here before. That’s because we have – former British Prime Minister Theresa May agreed a Brexit deal with the European Union almost a year ago. But that was rejected three times in the UK Parliament, and she was forced to resign.

Her successor, Boris Johnson, launched a huge effort to force the EU to reopen the deal and renegotiate the parts of it that his party, the Conservatives, didn’t like. They said it couldn’t be done, but on Thursday Johnson was in Brussels to sign the new deal with the EU.

Here’s what it all means.

What’s just happened?

In short, Boris Johnson has pulled off a huge political achievemnt. After being told repeatedly that he would be unable to renegotiate the Brexit deal that his predecessor Theresa May struck last year, the British Prime Minister has done exactly that.

It seemed unthinkable just weeks ago, but Johnson has convinced the EU negotiators that the only way he could sell a revised deal back home was if they removed something called the Irish border backstop, a mechanism that sought to keep an open border on the island of Ireland.

After weeks of tough negotiations, a legal text was produced just hours before the start of a two-day summit of European leaders in Brussels on Thursday.

What’s in the deal?

In many ways, the fundamentals of the deal are the same as that negotiated by May. The UK will still pay a £39 billion “divorce bill” to the EU, it will still guarantee the rights of EU citizens living in the UK, and a transition period will still last until December 2020, when all the current relationships with the EU will remain in place.

But here are some big changes, the most significant of which is the replacement for the backstop. The new Northern Ireland protocol is extremely complicated but, in short, Northern Ireland will be given special status, recognizing the unique history of the island of Ireland.

Under the new plans, Northern Ireland will stay aligned to EU rules in certain areas, while remaining fully part of the UK’s customs territory.

But while the deal says Northern Ireland will remain in the UK’s customs union, it also says that any goods that are “at risk” of ending up elsewhere in the EU – by making their way across the invisible border into the Republic of Ireland – will be subject to duties at the de facto regulatory border between Northern Ireland and the rest of the UK.

So, if a Northern Irish business wants to bring, say, a box of sugar from London to Belfast, it will have to pay EU tariffs when the box enters Northern Ireland.

If the business then sells the box of sugar in Ireland, the EU tariffs have already been paid. But if the business sells the sugar in Northern Ireland, it will be able to apply for a refund on the EU tariff it paid. Long story short: for Northern Irish businesses, the deal likely means new bureaucracy.

How it actually works will be highly controversial. But, it’s not the backstop, which should help Johnson succeed where May failed.

And what happened to the backstop?

Under the previous deal, the so-called backstop would have kicked in if the two sides failed to figure out a different way to avoid new border checks.

Since the new deal moves these checks from the island of Ireland to a de facto border between Northern Ireland and the rest of the UK, there is no need for a backstop.

But whereas the backstop would have been temporary, the arrangement laid out in this deal is permanent.

If Northern Ireland wants to change the deal in the future, it can do so through a simple majority vote, but it will have to go through a two-year cooling off period, during which the two sides would have to come up with solutions that would avoid a new border.

Is that it? Is Brexit done now?

If only. As stated above, it should help Johnson succeed, but it probably won’t. The main reason that May struggled to get her deal through Parliament was that it wasn’t supported by her Northern Irish allies, the Democratic Unionist Party (DUP).

The DUP believed that the backstop paved the way to Northern Ireland uniting with the Republic of Ireland and leaving the UK. For unionists, this is a non-starter. The problem is, Johnson’s backstop replacement arguably makes this a more likely outcome. And that’s why the DUP has said it would be unable to support Johnson’s deal in Parliament.

What is the DUP and why doesn’t it agree with the deal?

The DUP is a Northern Irish party which is very conservative and very pro being part of the UK. Their members, for example, oppose abortion and gay marriage, in contrast to their Conservative allies in London. The DUP’s priority throughout the whole Brexit process has been to ensure that Northern Ireland remains fully a part of the UK. It doesn’t matter if that happened through no deal or a very soft deal.

The problem for Johnson is that his deal puts a border in the sea between the UK and Northern Ireland. But it also contains something called the consent mechanism, which would give all Northern Irish political tribes the opportunity to consider its position at regular intervals. The DUP fear is that a republican majority could take control in years to come and force Northern Ireland out of its beloved Union.

Has Boris Johnson just done what Theresa May couldn’t?

Not really. In many respects, the deal is very similar to one offered by the EU and rejected by May early in the negotiating process. He still hasn’t secured the support of the DUP, and there are still skeptics within his own Conservative Party.

Hardline Brexiteers previously used the cover of the DUP to vote against May’s deal which they thought tied the UK too closely to Europe and betrayed the Brexit vote. Most of them seem to have fallen in line with this new deal, but there is no guarantee that all of them will.

And it’s extremely unlikely that any opposition parties will try to help dig Johnson out of his hole in Parliament. Which is a massive problem, given that he has a working majority of minus 40.

None of this, of course, will have been missed by those watching in Brussels. As one EU diplomat told CNN on Thursday morning, “our main concern is that Boris Johnson turns out to be Theresa May 2.0.”

Johnson has a long way to go before he can pop open the champagne. Having got through the EU summit, he must win over every one of his MPs, plus the 21 rebels he forced out when they voted for legislation that blocked a no-deal Brexit. On Saturday, he must stand before Parliament and with a straight face tell lawmakers they should vote for a deal which looks a lot like the one Johnson himself twice voted against.

Johnson’s optimism has taken him further than anyone expected. But, as ever with Brexit, at some point gravity will bring him crashing back down to Earth.

Ivana Kottasova contributed to this report.