Incoming Prime Minister Boris Johnson has 100 days to deliver on his promise of leading the U.K. out of the European Union.

The Brexit deal his predecessor struck with the bloc is “dead,” “defunct” and “needs to be junked,” he said during the leadership campaign. He’s vowed to renegotiate Brexit, and to quit the bloc on Oct. 31, with or without a deal to smooth the split.

So what’s his strategy, and what stands in his way?

Proposal: Renegotiate Withdrawal Deal

Johnson wants to scrap the deal Theresa May struck and restructure the whole negotiation that was agreed at the start of the process.

The most controversial element of May’s deal is the so-called Irish backstop, a fallback measure to ensure the border with Ireland remains open whatever future trading terms the two sides eventually agree on. He wants to postpone any talks about the border until after the U.K. has left the bloc, arguing they should be part of future trade negotiations. He also suggests using the 39-billion-pound ($49 billion) divorce settlement as leverage in the talks, making payment contingent on the terms negotiated.

Obstacles: The EU has repeatedly said it’s not prepared to renegotiate the Withdrawal Agreement, and it’s the only deal on the table. It’s also said a backstop must be part of any withdrawal accord, because it wants to lock in a guarantee that no hard border will emerge. The exit bill is also part of the divorce deal, and is due to be paid whatever trade terms are eventually negotiated. EU law dictates that Brexit should happen in two parts: First the exit negotiation, and once the U.K. has left, formal trade talks can begin.

Proposal: Leave on Oct. 31 “Do or Die’

Johnson has said that after May twice delayed Brexit, the U.K. must now leave the bloc on Oct. 31, “do or die,” whether a new deal has been negotiated or not. He’s refused to rule out suspending Parliament — known as proroguing — to achieve that.

Obstacles: The timetable is tight to negotiate an entirely new deal. And the U.K. Parliament is opposed to a no-deal Brexit. That was made clear again last week when Conservative rebels joined the opposition to pass an amendment aimed at preventing Johnson suspending Parliament. The EU side has indicated it would prefer another extension to a no-deal split.

Proposal: Standstill Period After Brexit

Johnson is seeking to negotiate a “standstill” period with the European Union after Oct. 31, during which there would be zero tariffs and zero quotas to “smooth things over for business” — much the same as the transition period negotiated by May. He said it would end “well before the next election,” which is due in 2022.

Obstacle: The EU has said any form of transition period is contingent on there being a withdrawal agreement, which must contain an Irish backstop.

Proposal: Use WTO rules if EU won’t cooperate

Johnson has said that if the EU won’t offer a new deal, the U.K. will be able to use Article 24 of the World Trade Organization’s General Agreement on Tariffs and Trade to ensure the terms of commerce remain the same after Brexit.

Obstacles: The clause Johnson cites applies to countries negotiating a trade deal rather than a country leaving a trading bloc. It would also require the agreement of the EU, and a clear timetable for the negotiation of a trade agreement — neither of which would be guaranteed in the event of an acrimonious no-deal Brexit. The WTO and EU have said this won’t work, and Johnson revealed a hazy understanding during the campaign as to why it would.