Large-scale migration occurred in south asia after 1947 primarily because of the

union blockade

explanation:

the union blockade in the american civil war was a naval strategy by the united states to prevent the confederacy from trading.

the blockade was proclaimed by president abraham lincoln in april 1861, and required the monitoring of 3,500 miles (5,600 km) of atlantic and gulf coastline, including 12 major ports, notably new orleans and mobile. those blockade runners fast enough to evade the union navy could only carry a small fraction of the supplies needed. they were operated largely by british citizens, making use of neutral ports such as havana, nassau and bermuda. the union commissioned around 500 ships, which destroyed or captured about 1,500 blockade runners over the course of the war.

worcester v. georgia, legal case in which the u.s. supreme court on march 3, 1832, held (5–1) that the states did not have the right to impose regulations on native american land. andrew jackson refused to enforce the ruling, the decision form the basis for most subsequent indian law in the united states.