For U.S. President Donald Trump, who is reportedly in a dark mood over the advancing Mueller investigation and fresh off a slew of Republican congressional losses in the midterm elections, a naval standoff between Russia and Ukraine comes at a particularly fraught moment in his presidency.

But for Russian leader Vladimir Putin, that makes this as good a time as any to see how far he can push Trump by asserting Russian supremacy in the disputed Sea of Azov.

Trump has cancelled his official one-on-one meeting with Putin scheduled for the G20, citing Russia's aggression on the Black Sea, but the Kremlin later said the leaders had a brief encounter at the summit on Friday.

Last Sunday, Russia seized control of three Ukrainian ships — two navy artillery boats and a tugboat — and detained two dozen Ukrainian sailors travelling along a vital maritime trade route.

"If Putin wanted to send a message that these are the things we're going to do — we're going to establish our sovereignty and domination of the Sea of Azov, and you're not going to be able to do anything about it — this is a good time to do it," said Jeffrey Mankoff, a senior fellow at Washington's Center for Strategic and International Studies.

He says Trump is in a particularly weak position at the moment. At home, there were the disappointing midterm results and special counsel Robert Mueller's investigation into Russian meddling in the 2016 U.S. presidential election continues to ensnare key members of Trump's inner circle. Abroad, the U.S.'s relationships with traditional allies are strained.

Trump's problems provide Putin with an opportunity to test the American response to his expansionist ambitions.

"I'm sure there have been other opportune moments over the year, and this is one of them," Mankoff said.

On Nov. 25, Russian ships fired on three Ukrainian vessels that were attempting to pass through a strait Russia has effectively controlled since it annexed Crimea in 2014. Since then, a freighter ship has blocked the only entrance in and out of the waterway. (CBC)

Last weekend, Russia's coast guard rammed the Ukrainian vessels and opened fire on them, injuring as many as six crew members in the first flash of direct armed conflict between the two countries since Russian forces annexed the Crimean Peninsula four years ago.

However, thousands of people in Eastern Ukraine have been killed in fighting between Russian-backed separatists and Ukrainian forces during that period.

Russia and Ukraine have long struggled over control of the narrow Kerch Strait, a major choke point between the Black Sea and the Sea of Azov, a joint sovereignty "internal" waterway that Ukraine depends on for shipping to the Ukrainian port of Mariupol.

'Moscow sees weakness'

Trump's initial refusal to blame Russian aggression for the tensions telegraphed weakness, according to Alina Polyakova, a foreign policy fellow at the Brookings Institution, a think-tank based in Washington, D.C.

"And where Moscow sees weakness, it also sees opportunity to undermine U.S. national security interests," Polyakova wrote in an op-ed this week.

The U.S. sees the preservation of a neutral zone between NATO and Russia's military as important for European security and stability.

A member of Russia's FSB security service escorts a detained Ukrainian navy sailor, right, before a court hearing in Simferopol, Crimea, on Nov. 27. (Pavel Rebrov/Reuters)

Critics noted Trump's cancellation of the formal Putin meeting came the same day court filings implicated him in business dealings with the Kremlin that potentially raise conflict-of-interest concerns. On Thursday, it emerged that Trump's former personal attorney, Michael Cohen, had struck a plea deal with Mueller and admitted to lying to the Senate about talks concerning a potential Trump Tower in Moscow that lasted well into Trump's 2016 presidential run.

Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting.... —@realDonaldTrump

With the advancing Mueller investigation consuming Trump's thoughts, what better moment to challenge American resolve and test the bounds of the U.S.-Russian relationship?

There's another relevant timing consideration for Putin. He might see an opportunity to destabilize the current Ukrainian presidential election. (For his part, Ukrainian President Petro Poroshenko, whose popularity is suffering, has declared martial law as a result of the inflamed tensions, and was suspected of trying to postpone the election.)

Prof. Henry Hale of George Washington University, who specializes in post-Soviet Eurasia, suspects the timing of Russia's aggression "is more related to the Ukrainian decision to send these ships there now."

"But Russia always has an interest in testing U.S. resolve on these questions," he said.

Like most Western powers, the U.S. recognizes Ukraine's sovereign right to access the Sea of Azov via the bottleneck. The initial act of provocation that seems to have escalated tensions was Putin's construction of a 19-kilometre bridge from the Russian mainland in the south to the annexed peninsula — a physical claim to the Crimean Peninsula. The bridge opened in May.

A May 2018 file photo shows a view of the new bridge across the Kerch Strait, linking Russia's Taman Peninsula with Crimea. (Associated Press)

"That is very provocative," Hale said. "This is another way of physically connecting the Crimean Peninsula to Russia. The bridge gives the Russians an opportunity to further lay claim to it, to use the bridge to block Ukrainian ships and normalize the idea that Crimea is part of Russia."

He points out Russia is prosecuting the Ukrainian sailors for unlawfully crossing Russia's border under domestic law, rather than treating it as a war-related confrontation.

Actions, not words

Jim Carafano, who worked on Trump's transition team and serves as a foreign and defence policy analyst with the Heritage Foundation think-tank, said too much attention has been paid to Trump's rhetoric on Russia rather than his administration's actions.

"The U.S. policy toward Russia is very tough," he said. The Trump administration has, for example, added $1.4 billion in funding to the European Deterrence Initiative to hold off Russian aggression. The president also approved sending lethal arms to Ukraine, a move that his predecessor, Barack Obama, never made.

Trump's refusal to meet formally with Putin at the G20 sends the right message, Carafano says.

"It would say, 'Dude, you pissed me off. I'm your last friend in America, and I'm pissed off.'"

But John Herbst, the former U.S. ambassador to Ukraine from 2003 to 2006, says cancelling a meeting doesn't punish the Kremlin in any meaningful way.

"Putin would be thrilled," he said, if that ends up being the strongest response from the U.S.

"What does that mean if he misses a meeting? It means nothing," Herbst said. "It means something if we sanction a major Russian bank. If we send anti-ship missiles to Ukraine. If we send fast patrol boats to Ukraine. Those are things that work. Those demonstrate that we truly are unhappy and will work to thwart Russian aggression."

Putin watches Trump as they gather for the group photo at the start of the G20 summit yesterday. (Pablo Martinez Monsivais/Associated Press)

Herbst agrees the Trump administration's policies on Russia have been "clearly better" than Obama's, but he also sees a pattern preceding the introduction of those tough policies.

"The president says something accommodating to Putin, and that sparks a reaction in his administration and makes people in Congress unhappy, and that results in sanctions. So you end up getting wiser, tougher policies."

Russia had long denied using its conventional forces against Ukraine. Now the Kremlin admits its forces were shooting at the Ukrainians, the first official and overtly hostile act by Russian military forces against Ukraine in years. That's alarming, says Herbst, who wants to see the West raise the cost of the Russian attacks with additional sanctions and possibly additional weapons supplies to Ukraine.

Whatever Putin's motivations, the timing is curious to Herbst, given that the Ukrainians sailed freely to the Sea of Azov less than two months ago, with no consequence. That successful journey in September was celebrated in Ukrainian media as a "risky operation," buoying hopes that it would mean continued access to a vital shipping lane.

"The person you have to look to as to why he chose to escalate this issue now is Mr. Putin," Herbst said.