As Hungary and Croatia close their borders, tens of thousands of migrants are now trapped in Serbia.BELGRADE — Hungary’s decision to seal its southern border has turned the spotlight of the migrant crisis onto Serbia, where images of columns of refugees fleeing conflict evoke all-too-recent memories. Only a decade-and-a-half ago, Serbia was an international pariah, shunned for its role in the Yugoslav wars, and perceived as the instigator of ethnic conflict and persecution. Now government and civil society have won praise for the compassion with which the current crisis has been handled. The government has wasted no time in drawing...