PRETORIA, June 8 -- For weeks, the confrontations had been spreading across South Africa's provinces: groups of men stopping other men on the street to demand proof of their nationality. By Sunday evening, the question of who has the authority to enforce South Africa's immigration laws had become the most urgent political matter in the country, and President Cyril Ramaphosa was standing at the Union Buildings to answer it.

The national address, announced only hours before it took place, was not merely a policy statement. It was a direct response to the anti-immigration movement March and March, whose leader Jacinta Ngobese-Zuma has given undocumented foreign nationals until June 30 to leave South Africa and has threatened to bring hundre...