WASHINGTON ― For most of the last century, with the exception of World War II’s partial thaw, America and its leaders have seen Russia as a dangerous rival, if not an outright enemy.

No significant presidential contender ― let alone president ― has ever made brotherly admiration for Mother Russia and its rulers a central theme of his or her campaign for the White House. Until, of course, Donald Trump.

The biggest question in the whole Trump saga is this: Why does he like Russians so much? The answer is to follow the money, specifically the rubles.

The Beltway world is agog over the news that a year ago June, the future president’s eldest son got as excited as a preppy in a Vineyard Vines store when told that a Russian lawyer with ties to the Russian government wanted to meet with him to dish officially dug dirt on his dad’s Democratic rival, Hillary Clinton. He agreed to a sit-down.

Donald Trump Jr., a big-game hunter by avocation but a total novice in politics, might have thought he was about to bag the Big One for dad. But he also may have broken the law by soliciting campaign aid from a foreign national or foreign government.

On Tuesday, Junior released the chummy, cringe-worthy (and perhaps criminal) emails related to the meeting, just a few minutes ahead of a story about them breaking in The New York Times.

In typical Washington fashion, Trump diehards and lots of others wondered who had leaked to the Times. Suspicion fell heavily on others in the Trumpian tribe, which is as brutally rivalrous as the Mafia ― all the more so because many of its members are now under investigation by congressional committees and special counsel Robert Mueller.

Everyone wants to know what this might mean legally for Junior and for two others in the meeting: Trump son-in-law Jared Kushner and then-campaign chairman Paul Manafort.

Pollsters and politicians are also eager to learn whether the latest revelation is (finally) going to shake the Trump base out of its stubborn insistence on sticking with their man. (The probable answer is no.)

But all of this misses a key aspect of that meeting in Junior’s modest (compared with Dad’s) office suite on the 25th floor of the Trump Tower in Manhattan. The underlying story here is that, beginning in the mid-1990s, Trump, his family and his business came to see Russians as the answer: a source of quick profits, big loans, international connections, clout and even jet-setting models for the Trump agency.

The alliance, so to speak, was an accident of history. When the Soviet Union collapsed in 1991, Russians with political connections bought up state-run companies at fire-sale prices and turned enormous profits. These new oligarchs needed places outside Russia to invest their rapidly gotten wealth ― much of it illegal or, at least, too glaringly gigantic for their own good.

Meanwhile, in Atlantic City USA, Trump’s dreams of empire were collapsing in the overbuilt casino industry. He was so close to ruin that he had to call on his own father to rescue him by buying more than a million dollars in chips.

As he climbed back to solvency by offloading debts onto new corporate entities, the respectable New York banks and investors wanted nothing to do with him. Trump, according to his own sons, came to rely heavily on Russian investments in real estate, golf courses and the like.

The Russians may well have saved him.

Whether that reliance led to any illegal activity ― say, money laundering through real estate purchases and other investments by the newly rich of Russia ― lies at the heart of Mueller’s deep-dive investigation into the motivations of Trump and those around him.

Mueller has hired lawyers and investigators steeped in the complexities of global money movements with this history in mind, according to two attorneys who know him well. “It’s follow-the-money,” one said. “And with Trump that will take a lot of expertise.”

As in any criminal conspiracy probe, it’s crucial to uncover the motivations of the suspects ― Trump and his cronies.

The June 2016 meeting involved four men and one woman who together had decades of experience in the money, real estate and power games of Moscow: Junior; Manafort, who had worked for pro-Russian forces in Ukraine; Kushner, whose family sought Russian investments; British entertainment promoter Rob Goldstone, who represents a Russian pop star with an oligarch father; and Russian lawyer Natalia Veselnitskaya, who got the meeting via a connection with said oligarch and Vladimir Putin buddy who had done business with Trump.

Could it be that Trump’s team was just too ignorant and uninformed to know that it was illegal to accept the help of a Russian with governmental ties whose information purportedly came through “the Crown prosecutor of Russia” and “official documents”? Or did they feel comfortable dealing with powerful Russians because they had done so for years? Or did they fear some kind of pressure if they didn’t accept this “help” from old friends?

To follow the money and assess the motivations, Mueller will almost certainly (eventually) subpoena Trump’s income tax returns, which, contrary to recent tradition among presidential candidates, he has refused to release to the public. While the special counsel is at it, he may as well ask the judge for the tax returns of others, including Donald Jr., Eric and Ivanka.

Since the story of the Trump Tower meeting broke, Donald Sr. has kept his distance, refraining for once from tweeting a defense and merely issuing a statement that his son is a “high-quality person.”

If you know the family story, there is something sadly appropriate about the distance. Junior, an affable if somewhat hot-tempered sort, was not close to his father for years after the elder Trump divorced his mother. More recently, Junior has not been the favored child; that distinction belongs to Ivanka. In Trump Tower, Ivanka’s office was on the 26th floor, where dad worked; Don’s was one floor below.

Trump’s spokespeople insist that he never knew about the original promise of that 2016 meeting. But if he had, he probably would have said what Junior said in an anticipatory email:

“I love it.”