British lawmakers have debated banning his entry to their country. A former Australian prime minister said the prospect of his presidency made him "tremble." Piñatas in his image, complete with his signature coif, have become all the rage in Mexico.

The unorthodox march of Donald Trump toward the Republican nomination has transfixed audiences from London to Latin America. The world has looked on with confusion, consternation, and a few congratulations at the prospect that a brash billionaire with no political experience, who says whatever he wants, could possibly find his way to the White House.

Any US presidential race is hot-ticket international news. But this one comes at a time of deep global insecurity. Mr. Trump's status as an unknown quantity feeds nations' worries about everything from the continuation of their trade deals to military ties.