Why does President Trump seem so fond of Russia and its strongman leader, Vladimir Putin?

We asked Michael Isikoff of Yahoo News, co-author of the new bestseller, “Russian Roulette: The Inside Story of Putin’s War on America and the Election of Donald Trump.” And the answer may be pretty simple: It’s all about business.

“Everybody keeps looking for the smoking gun, the foolproof evidence that explains Trump’s Russia problems,” Isikoff tells Yahoo Finance. “But much of the evidence is hiding in plain sight.”

As a real-estate developer and head of the Trump Organization, Trump tried for years to build a Trump Tower in Moscow, just as he had in New York, Chicago, Vancouver, Istanbul and other marquis cities. By 2014, he was actually getting close. Trump had made the right connections during a 2013 trip to Moscow, where the Miss Universe pageant, which he owned, was held. He knew Putin’s personal approval would be necessary to build his tower. And by early 2014, his company had signed a letter of intent with Moscow authorities to go ahead with the project.

Trump’s daughter Ivanka, who was then one of the company’s top executives, was actually in Russia scouting for sites in February of that year, when Russia, under Putin’s order, annexed Crimea. Then Russian troops invaded Ukraine. Europe and the United States responded by imposing tough new economic sanctions on Russia—which directly affected a Russian bank Trump had lined up to help finance his Moscow tower.

“It was the sanctions that killed his Russia deal,” Isikoff says. “It helps explain a lot.”

As a candidate, Trump criticized those sanctions, without ever mentioning that they directly affected his business dealings. And as president, he has been strangely solicitous of Putin, bucking most of the U.S. national-security establishment and the orthodoxy of his own Republican party. Trump, for instance, called Putin recently to congratulate him for winning an “election” widely viewed as completely rigged. Trump’s own national-security advisers had strongly advised him not to congratulate Putin.

Trump persisted in his efforts to get a Moscow deal, even after declaring his candidacy for president in 2015. Later that year, Trump lawyer Michael Cohen—now embroiled in the Stormy Daniels controversy—spearheaded another effort to build a Trump tower in Moscow.

“The public knew nothing about it,” Isikoff says. That project never got off the ground, either.

While pursuing the Moscow deals, however, Trump and his company—including family members—developed a number of Russian contacts who circled back once Trump was a presidential candidate. Those were the Russians who offered “dirt” on Hillary Clinton, or helped set up meetings on the matter. A few are known confidantes of Putin. “If you want to understand what happened at these meetings,” Isikoff says, “you have to understand these business relationships.”