Yugoslavia and WWI?

It was a compromise of effectiveness. Italy wanted Croatia, which didn't have the resources to defend itself, but had the manufacturing and scientific infrastructure. Slovenia had the farming resources, but little else. Serbia had the military capability. Since there was (is) both ethnic and cultural ties that overreached the region, it was decided to put the whole area together. The thinking was that a large nation would be more stable than several small ones. Plus there were disputes over the 'internal' boundaries that England (and to a lesser extent France and the US) hoped would become non-issues.