Tensions between Russia and Ukraine have reached their most dangerous moment in years — one that has the potential to ignite a new phase in the deadly conflict.

Here’s what happened: On Sunday, Russian ships fired on three Ukrainian vessels in the Kerch Strait — a critical passage connecting the Black Sea to the Sea of Azov — injuring at least six sailors. Moscow’s crew has since boarded Kiev’s two warships and one tugboat, detaining more than 20 sailors, and even placed a freighter ship length-wise along the only entrance in and out of the strait.

Kiev is unhappy. Ukrainian President Petro Poroshenko has announced that he wants to declare martial law for at least the next 30 days, allowing his country to mobilize forces more quickly.

Meanwhile, the UN Security Council — where Russia is a permanent member with the authority to singlehandedly veto any measure — met on Monday morning to discuss the situation.

The standoff centers on the continued dispute over Crimea. Russia annexed the peninsula from Ukraine nearly five years ago and has since tightened its grip on it. In May, for example, Russian President Vladimir Putin inaugurated a 12-mile bridge over the Kerch Strait connecting Crimea to mainland Russia despite Ukraine’s protests.

The Kremlin’s goal is to bring the land and waters around Crimea completely under Russian control. That’s partly why it fired on the Ukrainian ships: as a way of asserting its dominance over the waterway.

So far, only a war of words has broken out, with each side blaming the other for what happened. Poroshenko called the move “an act of aggression;” Russia’s foreign ministry said Ukraine was responsible for the “well-thought-out provocation.” But the skirmish has the potential to grow into an even bigger fight if Russia uses warships and warplanes to keep Ukrainian vessels out of the Sea of Azov.

The dispute could also become a major test for President Donald Trump as he decides how — or if — to respond to Russia’s aggression. Pressure is already mounting from Congress and analysts for the US to respond forcefully against Russia, or risk letting Moscow get away with it.

“This is Russia taking advantage of Donald Trump for its own gain, operating under an, ‘I can do what I want and no one will stop me’ attitude,” Evelyn Farkas, a top Pentagon official for Ukraine and Russia from 2012 to 2015, told me.

The Trump administration has so far offered mixed signals, with UN Ambassador Nikke Haley saying that Moscow’s moves are “another reckless Russian escalation” and Trump claiming that “We don’t like what’s happening either way” — seemingly not putting the blame on the Kremlin.

If Trump chooses not to respond more forcefully, then it’s possible Russia will only escalate its actions in the Kerch Strait and possible NATO territory in the future.

Why Russia is doing this now

Russia and Ukraine have been at war since early 2014, when Moscow’s forces invaded Eastern Ukraine. That conflict roils today whereas Russian agents, some uniformed and others disguised as civilians, try to seize that territory away from Kiev’s control.

Russia’s seizure of Crimea from Ukraine is Putin’s greatest achievement in the conflict, as it gave him access to new ports and waterways, all while taking land away from Ukraine. And to solidify his gains, Putin is trying to bring the bodies of water around Crimea further under the Kremlin’s control.

The water around Crimea matters. For one, more than 80 percent of Ukraine’s exports go through the Sea of Azov, which makes it vital to an independent Ukraine. It’s also an important fishing hub that provides both food for the region and an economic boon.

“Russia has been engaged in a policy of creeping annexation in the Sea of Azov ever since it completed construction of the Kerch Bridge,” Michael Carpenter, a top Pentagon official for Ukraine and Russia from 2015 to 2017, told me.

Russia started by harassing Ukraine’s commercial ships, Carpenter said. Then it grew its naval capabilities in the region. Now it’s taken the next step: firing on and boarding Ukrainian ships while denying them further access to the waterway.

But why escalate?

Analysts I spoke to highlighted three main reasons why Russia likely chose to take this next step right now.

The first is that Ukraine’s Orthodox Church said last month that it would separate from Russia’s. That’s a big deal, as Moscow used the church to spread its propaganda to devout Ukrainians, explained Farkas, the former Pentagon official.

Russia has counted on the church’s support for the past three centuries; now, Moscow has lost a major way to spread its influence. That’s a problem for Putin, as the schism allows Ukraine to become more independent of Russia — right as he’s trying to bring it further under Moscow’s control.

Poroshenko celebrated the move in October by saying it was “absolutely necessary to cut off all the tentacles with which the aggressor country operates inside the body of our state.”

The second reason is that Poroshenko is up for reelection in March. The race is already tight, and taking access to the Sea of Azov away from Ukraine might hurt him by negatively impacting the economy. However, Poroshenko may gain voter support for staunchly standing up to Putin’s aggression.

Finally, experts said that Putin typically uses aggressive military tactics as a way to improve his declining approval rate. Putin has promised to turn his country into a global economic powerhouse and improve the lives of everyday Russians — but Russia’s economic growth is slowing, and Russians aren’t happy.

So it may be a shrewd political move on Putin’s part to attack Ukraine and boost his image at home.

The big question now is whether the US — along with others in the international community — will seriously respond to Moscow’s aggression. As of now, that looks unlikely.

“More military aggression against Ukraine is coming”

There’s little Ukraine can do on its own to break Russia’s blockade, even though a 2003 treaty allows it access to the Sea of Azov. “Ukraine has almost no naval capabilities with which to defend itself,” Carpenter told me, although it still plans to move some of its assets to the area.

That’s why many analysts say Ukraine needs help — and that Russia needs a severe reprimand.

In terms of aiding Kiev, Carpenter says the US should give Ukraine radars to better surveil the waters its ships are in, as well as land-based anti-ship missiles to better defend itself. The Trump administration already increased the amount of lethal support the US is providing to Ukraine, but it’s unclear if it would be willing do so again over this incident.

But most experts I talked to said they’d like to see the US and Europe sanction Russia over its actions — for instance, freezing the assets of a Russian bank, targeting individuals involved in the decision to fire on Ukraine’s ships, or even revoking the visas of some Russian officials. The US and European countries could also bar Russian ships from docking in their ports for a while.

The idea is to convince Putin that the blockade will cost him too much. If no one does that soon, “it means that more military aggression against Ukraine is coming,” Alina Polyakova, an expert on European security at the Brookings Institution, told me.

It’s also possible, some experts say, that a weak response to this incident would give Russia the confidence to carry out similar aggressive moves in Lithuania, Latvia, or Estonia — all of which are NATO members. If that happened, then the US and its European NATO allies would be treaty-bound to defend those countries and go to war with Russia.

So far, the Trump administration’s response has been tepid — but that doesn’t necessarily mean it won’t take some kind of action.

In fact, despite Trump’s desire to improve relations with Russia and his seemingly cozy personal relationship with Putin, his administration has sanctioned Russia many times, most recently over its attempted murder of a former Soviet spy in the UK this year. The US also has sanctions in place after Russia’s annexation of Crimea, and UN Ambassador Nikki Haley said on Monday that they will stay in place.

But if the Trump administration decides to let Russia slide on this latest move, experts warn that Sunday’s skirmish may be the beginning of a whole new phase in the Russia-Ukraine war — and perhaps an even bigger one.