Almost exactly 13 years ago, American leadership brought an end to Bosnia's three-and-a-half-year war through the Dayton peace agreement. Today the country is in real danger of collapse. As in 1995, resolve and transatlantic unity are needed if we are not to sleepwalk into another crisis.

Bosnian Serb Prime Minister Milorad Dodik, once the darling of the international community (and especially Washington) for his opposition to the nationalist Serb Democratic party, has adopted that party's agenda without being tainted by their genocidal baggage. His long-term policy seems clear: to place his Serb entity, Republika Srpska, in a position to secede if the opportunity arises. Exploiting the weaknesses in the country's constitutional structure, the international community's weariness and EU inability to stick by its conditionality, he has, in two years, reversed much of the real progress in Bosnia over the past 13, crucially weakened the institutions of the Bosnian state, and all but stopped the country's evolution into a functioning (and EU-compatible) state.

Dodik's actions have been fuelled by Russian encouragement and petrodollars. In addition his rival, the senior president of all of Bosnia-Herzegovina, Haris Silajdzic, has stressed the need to abolish the two entities that make up Bosnia, to create one non-federal country. Dodik professes to respect Dayton and Silajdzic wishes to revise it, but both men are violating its basic principle: a federal system within a single state. This toxic interaction is at the heart of today's Bosnian crisis.

As a result, the suspicion and fear that began the war in 1992 has been reinvigorated. A destructive dynamic is accelerating, and Bosnian and Croat nationalism is on the rise. The recent local elections gave a fillip to nationalist parties.

This tipping point is the result of a distracted international community. While the Bush administration largely turned its back on Bosnia, the EU became deeply engaged; EU membership has been the critical lever for pressing reforms in Bosnia since it was made policy in 2003. But the EU did not develop a coherent strategy, and by proclaiming progress where it has not been achieved, the EU has weakened not only its own influence in the country, but also the Office of the High Representative (OHR) and the international military presence (the European Union Force, Eufor, which succeeded Nato) the drivers of progress in Bosnia since Dayton.

The degeneration of the OHR's influence coincided with the withdrawal of the US military and the hollowing-out of Eufor, which now has little in the way of operational capacity. Despite the danger signals, France and Spain apparently want to pull the plug on Eufor altogether before the end of the year, seemingly to prove the purely technical point that EU missions can end.

The EU, fixated on a still undefined "transition" from OHR to an EU-centred mission, seems intent on emptying its toolbox before it knows what tools it will need to enable Bosnia's transition. It failed, for example, to back its man on the ground, the able Slovak diplomat Miroslav Lajcak, at a crucial moment, fatally undermining his authority.

Like Dodik, Russia is exploiting weak EU resolve, making trouble for the US and EU where possible. Yet Moscow's equities in Bosnia pale in comparison to those of the EU or US. Their attempts to close the high representative's office, regardless of whether the job is done, must be rebuffed. It has to remain open - or a similarly strong organisation set up - until the conditions for the transition to a more normal EU presence are met. The US, lame duck or not, must re-engage.

Javier Solana, the EU's foreign policy chief, should initiate an independent study tasked to produce a new transatlantic policy, backed by Washington's full engagement and strong EU conditionality, which can lead to deeper and broader international involvement in Bosnia. A collapse of the Dayton peace agreement would be an unnecessary and unwanted additional problem for the new White House administration.

Post-Irish referendum, the EU's foreign policy will be, above all, a Balkan policy. Attention has recently focused on Kosovo. But Bosnia has always been the bigger and more dangerous challenge. The country's decline can still be arrested, provided the EU wakes up, the new US administration gets engaged, and both renew their commitment to Bosnia's survival as a state, by maintaining an effective troop presence and beginning the process of strengthening the international community's approach long-term, including finding ways to untie Bosnia's constitutional knot.

It's time to pay attention to Bosnia again, if we don't want things to get very nasty quickly. By now, we should all know the price of that.

• Paddy Ashdown was the international community's high representative and EU special representative in Bosnia and Herzegovina from 2002 to 2006. Richard Holbrooke was the chief architect of the 1995 Dayton peace agreement