STOCKHOLM – The next 18-24 months are likely to decide the shape of Europe for decades to come, and the United Kingdom has now started the clock on that process. Reelected with a resounding – and entirely unexpected – majority in the House of Commons, Prime Minister David Cameron must now use his increased mandate to set out an EU reform package that is attractive to all member states.

In recent years, the tail has tended to wag the dog in the UK, with Cameron kowtowing to the fanatically anti-European wing of his Conservative Party, if only to hold the pro-withdrawal UK Independence Party at bay. But now that his own authority has been strengthened significantly by his victory, with the UKIP emerging as the election’s biggest loser, he can now step forward as the pragmatic but committed European that he truly is.

In a series of speeches over recent years, Cameron has spoken about a European reform agenda centered on increasing the EU’s competitiveness and improving its institutions’ transparency. In the wake of Russian revanchism and the mayhem spreading across the Middle East, were Cameron to speak today of the changes that Europe needs to make, I would hope that he would add his support for more effective common foreign and security policies.

If Cameron sets out such a reform agenda at the European Council in June and is prepared to listen as well as to talk, he could set in motion a process that benefits all of Europe. Then, it will be primarily up to EU Council President Donald Tusk, under the Luxembourg, Netherlands, Slovakia, and Malta presidencies of the EU over the next two years, to move a reform package forward by early 2017.

This will be a process in which the EU’s 28 member states, rather than the European Commission, must be in command. Only by appealing to and involving the EU’s national political institutions can EU reform succeed. Next year should be a period of intense debate on a reform package that, when put together, will, it is hoped, be agreed by all of the EU’s members, because Cameron needs to hold his promised in-or-out referendum on the EU before the UK takes over the rotating presidency on July 1, 2017.

At the moment, opinion polls indicate that the UK electorate would vote for continued EU membership. Then again, no opinion polls predicted that the general election would result in a majority Conservative government. So no one should be under any illusion about the risks inherent in any British referendum on EU membership.

Of course, the EU is not powerless to influence the outcome. The Union can do its part in the coming 18 months by demonstrating its ability to deliver not only a potent reform package, but also implement other key policies, such as the Transatlantic Trade and Investment Partnership with the United States and the Digital Single Market. Success in such areas, and the economic benefits they will bring, will make leaving the EU even more unattractive for the UK.

But a UK decision to leave, should it come to that, would initiate a painful and complicated process of negotiating an exit and agreeing on some sort of new relationship. There would be no attractive options, and the result – regardless of how much goodwill both sides bring to the talks – would leave both the UK and the EU visibly diminished, not least on the world stage.

Moreover, it would be naive to expect that the rest of the EU could just go on as before. On the contrary, British withdrawal would likely inspire similar moves in other countries, with the risk that the EU, already weakened, might begin to fragment. And, given his current efforts to divide Europe, one can be sure that Russian President Vladimir Putin would do all that he can to encourage, and finance, such a split.

During this period, the EU would also have to address the ongoing challenges to its eastern neighbors, particularly Ukraine, posed by Putin’s revisionism, as well as the meltdown of much of its southern neighborhood in the Middle East and North Africa. In this context, a weakened and fractured Europe – unable to confront threats stemming from its immediate environment – would be a more dangerous place to live.

Cameron’s remarkable victory should be viewed as an opportunity to launch a renewed and reformed EU in the next two years. The UK’s European partners expect Cameron to frame the debate that must now begin if a truly stronger EU – one that can face up to its future and its future challenges – is to emerge. But there is also the possibility of it all going terribly wrong. In these dangerous times, the consequences of Europe’s disintegration must not be underestimated.

Carl Bildt was Sweden’s foreign minister from 2006 to October 2014, and was Prime Minister from 1991 to 1994, when he negotiated Sweden’s EU accession. He is a member of the World Economic Forum’s Global Agenda Council on Europe.

Copyright: Project Syndicate.