Croatia, Hungary, and Yugoslavia are embarking on an ambitious project to create a cross-border nature preserve of at least 250,000 acres at the confluence of the Danube and Drava rivers, Central Europe’s largest wetland. Hungary and Croatia will sign a cooperation agreement today, and unofficial talks are underway with Yugoslav biologists and ecologists. The area — which is home to wild boar, otters, and more than 270 bird species — was the site of a raging war less than a decade ago, and this cooperative effort between nations is expected to help promote peace as well as conservation. The World Bank and other institutions have pledged nearly $10 million in loans and grants to help manage the park and boost tourism by constructing bicycle routes and observation towers.