PARIS — A boat crowded with hundreds of Africans sailing across the Mediterranean after being turned away by Italy this week has exposed anew the shaky fault lines in Europe’s approach to the migrant crisis.

On Sunday, Italy’s new far-right interior minister, Matteo Salvini, ordered the Aquarius, a rescue ship operated by humanitarian groups, to stop 35 nautical miles off the coast of Italy, refusing to let it dock.

The ship is now on its way to Spain, which showed up its neighbors by solemnly announcing that it would “respect its international engagements” and accept the boat after Malta, too, refused it, and France stood idly by.

Brussels, the seat of the European Union, looked on in relative silence. There was no common policy to receive the Aquarius and no authority to impose one if there were.