LONDON — It is hard, if not impossible, to recall a time when Europe’s view across the Mediterranean has been so clouded by conflagration in what seems a redrawn crescent of crisis from North Africa to Turkey.

And, some analysts have come to conclude, it is equally difficult to remember a moment when European leaders have seemed more distracted in rising to the challenge beyond their own frontiers.

The powerful seem hamstrung by the ascendancy at home of rightist insurgents whose appeal to disaffected citizens feeds on opposition to migrants in their midst. A decision by British voters to leave the European Union has only deepened the introspection.

The term “arc of crisis” was coined at the height of the Cold War by Zbigniew Brzezinski, President Jimmy Carter’s national security adviser, who spoke in 1978 of an area “stretching along the shores of the Indian Ocean, with fragile social and political structures in a region of vital importance to us threatened with fragmentation.”