On Monday, Croatia becomes the 28th country to join the European Union, its accession seen in Brussels as a triumph of the European project. An authoritarian country riven by conflict, where war criminals once operated with impunity, has been transformed. Despite the euro crisis, the Union’s “magnet effect” is still widely regarded as having the capacity to change wayward nations into states that follow liberal European norms.

Yet, while accepting that Croatia has made great strides in overcoming war and authoritarianism, there are compelling reasons why the country’s admission could be premature. Widespread political and economic corruption persist, and its courts often show an overly lax attitude toward due process. The biggest threat, though, is the country’s economic weakness, which may make it very difficult for Zagreb to sustain an already beleaguered reform agenda.

The fact is that the Union may well be about to repeat the mistakes of the last round of accessions, jeopardizing Croatian hopes for a better future, diminishing membership prospects for other Balkan states and Turkey, and may be even stalling the bloc’s enlargement efforts for the next decade or more.

When Bulgaria and Romania joined in 2007, the European Commission believed that both countries had overcome the corrupt practices of the past, that there would be no backsliding. But once the two countries were admitted, Brussels lost the leverage it had held during the application process. Incentives to comply with E.U. obligations eroded. Transparency International rates Bulgaria as the 75th most troubled nation on its Corruption Perception Index. Romania is rated at 66, Italy 72 and Greece 94. Croatia barely comes in ahead at 62.