The Syrian cease-fire is "hanging by a thread," as U.S. Secretary of State John Kerry puts it. Russia is sending an aircraft carrier speeding toward Syria's coast and the U.S. has mounting evidence that Russian warplanes bombed a humanitarian convoy carrying food and medicine just outside Aleppo, killing medical workers and patients.

These events and others should bring renewed scrutiny to Republican presidential nominee Donald Trump's cozy relationship with Russian leader Vladimir Putin, who must be delighting in a potential U.S. president who is so gullible and easily manipulated.

Trump's open affinity for Putin, his admiration for a style of governing that is just short of dictatorial and his possible financial ties to the former Soviet Union could make this country uniquely vulnerable to a longtime adversary. This is where Trump's continued refusal to release tax returns becomes more than a curiosity. His son, Donald Trump Jr., has boasted of the family's business dealings with Moscow, stating in 2008 that "Russians make up a pretty disproportionate cross-section of a lot of our assets … we see a lot of money pouring in from Russia."

Trump's campaign has included staffers with longtime ties to Moscow. Former campaign manager Paul Manafort, whose client list reads like a Who's Who of small-time dictators, was a longtime adviser to the Putin-backed Ukrainian president who was ousted in 2014 and has been linked to millions of dollars in undisclosed cash payments from pro-Russian groups.

Manafort worked behind the scenes during the Republican National Convention to remove a platform amendment that would have offered military aid to Ukraine to repel Russian and rebel forces. Tellingly, Trump last week rebuffed Ukraine President Petro Poroshenko, declining a request to meet with him. Poroshenko did meet with Democratic nominee Hillary Clinton, who expressed support for the country, as has been policy for both Democratic and Republican leaders in the U.S.

Trump's actions, wittingly or unwittingly, play right into Putin's aims to weaken NATO, formed primarily to safeguard Europe against Soviet aggression. Trump has placed a low priority on cooperation among traditional Western allies, signaling that he would put relations on a more transactional basis. His call for Russia to hack the e-mail of his rival may have been a joke to him, but it was shocking to many who would consider it a near-treasonous invitation to interfere in a U.S. election — particularly when there is evidence Russia may be doing just that.

Those who think Russia is relatively harmless compared to the old Soviet Union should think again. Mitt Romney took unfair criticism in 2012 when he warned of a revanchist Russia. He detected, even before President Obama, signs that Putin was about to launch the most muscular effort to re-establish Russia's standing on the world stage in decades. Within two years, a revitalized Russian military had annexed Crimea from Ukraine and made incursions into its Donbass region, where conflict continues. Trump has said he is willing to look at whether the U.S. should recognize the Russian annexation.

Trump has repeatedly expressed admiration for Putin's "strong control" of his country. What exactly does he admire? In Putin's Russia, political protesters face heavy fines and years in forced labor camps. Those who oppose Putin also have a nasty habit of turning up dead. He has cracked down hard on Chechen separatists, Islamic insurgents in the North Caucasus and, oh yes, gays.

Russia has proved over and over that the U.S. is right to hold it at arm's length, even as it negotiates. Trump so far looks like an easy mark for a nation led by a former KGB agent. In Monday's debate, he should make clear the extent of his financial ties and offer greater clarity about where he would take U.S.-Russian relations.