It is a dramatic achievement for Croatia, which when I lived there in 1998 was still recovering from conflict and getting to grips with the social and economic changes needed to transform it into a modern European state. The EU's enlargement rules were changed after the 2004-07 enlargement to make this probably the toughest process that any candidate country has ever successfully gone through. I'm very glad to have assisted Croatia's Chief Negotiator, Vladimir Drobnjak, during the process.
There was an audible intake of breath when the Croatian prime minister, Jadranka Kosor, went up to sign the treaty this morning - neither at the beginning nor the end of the process, but in the middle, Hrvatska coming between France and Italy. She is on borrowed time, having lost last weekend's election, but it was a moment when local divisions, whether in Zagreb or Brussels, were put aside and the EU for once remembered the big picture of unity across the continent.
The EU leaders then went back to resolve other issues such as Serbia's troubled European aspirations, setting a date for Montenegro to begin membership negotiations, energy policy and admitting Romania and Bulgaria to Schengen. But from today on, Croatian officials will sit in all EU meetings along with the 27 current member states. I hope it won't be too long before other countries in their neighbourhood join them.