Moscow CNN  — 

The four-and-a-half-year battle for eastern Ukraine has claimed over 10,000 lives, battered the Ukrainian economy and left Russia hit with crippling sanctions. Now, the mostly hidden conflict between two former Soviet Republics is threatening to explode into open war.

On Sunday, Russia seized three Ukrainian navy ships and detained 24 sailors in a strategic waterway that links the Azov Sea with the Black Sea. Kiev responded by imposing 30 days of martial law, and President Petro Poroshenko warned darkly that a full-scale Russian invasion might be imminent.

“Reconnaissance data suggest an extremely serious threat of a land-based operation against Ukraine,” Poroshenko said, in calling for the emergency measures. “Ready at any moment for an immediate invasion of Ukraine. A rifle hanging on the wall will go off sooner or later.”

So is war imminent?

Russia and Ukraine, for all intents and purposes, are already in an undeclared war. Kiev’s troops have been battling Russian-backed separatists in the eastern Donbas region who have received covert military support from Moscow. Russia seized and annexed the Black Sea peninsula of Crimea following a lightning operation led by special-operations troops in 2014.

But the Kremlin has maintained a veneer of deniability. While the US, its allies and Ukraine say Russia has sent heavy weaponry and even frontline units – Kurt Volker, the Trump administration’s point man on Ukraine, has said the separatists are “100% under Russia’s command and control,” – Moscow says the war in Ukraine’s east is an uprising driven by local grievances.

“We’ve never said there are no people there who deal with certain matters, including in the military area, but this does not mean that regular Russian troops are present there,” Russian President Vladimir Putin said in a 2015 press conference.

That, until now, was the closest the Kremlin has come to an admission of its involvement in a shadow war in Ukraine.

On Sunday, that conflict came back into view as Ukrainian and Russian vessels engaged in an open clash at sea. Russian officials have cast the incident as a legitimate defense against an incursion into its territorial waters, saying Ukraine was directing a “provocation” against Moscow. Ukrainian authorities say the Russians were deliberately trying to provoke an overwhelming military response, pointing to the heavy deployment of Russian military aircraft and warships in the incident.

Without predicting Putin’s next move – or having access to military intelligence – it’s easy to see reasons for Ukraine to treat this as a national emergency.

For starters, there’s the Ukrainian port of Mariupol on the Azov Sea. It’s an economic conduit for Ukraine, but equally important, it’s a potential link between separatist-controlled territory in the east and Russian-controlled Crimea.

In a 2015 analysis, Steven Pifer, the former US ambassador to Ukraine, observed that Mariupol was part of a potential “land bridge” to Crimea.

“Seizing Mariupol would be an important step to make a frozen separatist-occupied Donbas economically viable,” he wrote. “Mariupol is the port through which much of the steel and other industrial products of the Donbas are exported.”

“Mariupol is also key if the Russians desire a land bridge to Crimea. While taking the land bridge would be doable for the Russian army, holding it would prove costly,” Pifer added.

Different facts are on the ground today. Earlier this year, Russia opened a 19-kilometer bridge linking Russia’s Krasnodar region with the Crimean peninsula. The bridge spans the Kerch Strait, which Russia temporarily closed off during Sunday’s flare-up at sea.

Further complicating matters is a presidential election coming up early next year in Ukraine. Poroshenko is lagging in the polls. Some observers have questioned the timing of the introduction of martial law, which was initially proposed to last 60 days and might potentially have caused a delay in elections scheduled for the end of March.

The current regime of martial law – introduced in 10 Ukrainian regions located along Ukraine’s borders with Russia and Moldova, as well as the coast of the Black Sea and Azov Sea – is limited to 30 days, and Poroshenko has said that the decree will not restrict basic civil liberties.

The Kremlin is already making hay out of the martial law decree. Kremlin spokesperson Dmitry Peskov said Tuesday that Ukraine’s decision had a “barely camouflaged” political agenda, although he added that the move was an internal matter for Kiev.

But poll ratings and domestic Russian politics are also a matter of concern to the Kremlin. Putin’s usually sky-high approval ratings have slipped in recent months amid an uproar over an overhaul of the pension system.

Whether the escalating confrontation with Ukraine bolsters or dents Putin’s ratings, then, remains to be seen.