New Delhi, Aug. 5 -- The men shared bear hugs, showered praise on each other and made appearances side by side at stadium rallies - a big optics boost for two populist leaders with ideological similarities. Each called the other a good friend.

In India, the bonhomie between Prime Minister Narendra Modi and U.S. President Donald Trump was seen as a relationship like no other. That is, until a series of events gummed up the works.

From Trump's tariffs and India's purchase of oil from Russia to a U.S. tilt toward Pakistan, friction between New Delhi and Washington has been hard to miss. And much of it has happened far from the corridors of power and, unsurprisingly, through Trump's posts on social media.

It has left policy experts wonderi...