More than the offensives that retook so much land in Septemberthe recent recapture of Kherson marks a clear decision point for Ukraine.

Russian forces are now definitively on the defensive – their priority is not to take or retake ground, but to try to hold what they have. To achieve this, they are developing defensive systems in depth and desperately trying to prepare barely trained conscripts for high-intensity combat. The immediate priority for Ukraine is to keep the Russians off-balance so as not to allow them to consolidate their positions or train and prepare those reinforcements.

We can now see with some degree of confidence where the focus of both sides will be over the next months: Crimea. Of all the areas taken from Ukraine since 2014, Crimea carries by far the most significance. In a wide-ranging and fascinating interview in September this year, Ukraine’s commander-in-chief, General Valerii Zaluzhny, stated that Crimea is Russia’s “centre of gravity” – the key to the war.

This is certainly also the case for the Ukrainians. Their recent coups de main – such as the serious damage to the Kerch Bridge and the maritime drone attack on the Black Sea Fleet – only reinforce this. With the initiative and momentum with them, the question now is how Ukraine will deal with Crimea’s recapture. The answer will dictate the shape of the war.

Strategically vital peninsula

Crimea was transferred to Ukraine in 1954 by Soviet leader Nikita Kruschev. At this point, Ukraine was a wholly owned subsidiary of Moscow and its independence was considered an effective impossibility. Crimea had been central to Russia’s grand strategy since its capture from in 1783 from the Ottoman Khanate that ruled there. Sevastopol on the south coast contains Russia’s only significant warm-water naval base, giving it year-round access to the Mediterranean and all points beyond.

It was so vital to Russian interests that after Ukrainian independence in 1991 the naval base at Sevastopol was leased from Ukraine – an agreement that was cancelled by Vladimir Putin after Russia’s 2014 annexation of Crimea. There are several other significant military bases on the peninsula. For example ballistic missile early-warning radar has been built just outside Sebastopol, part of a much larger system which surrounds Russia.

But most importantly, it is the only major part of Russia’s occupied territories thought to have a majority ethnic Russian population. For centuries it has been a prime Russian summer holiday spot. Now, property prices are falling and the Ukrainian army is, if not quite at the gates, uncomfortably close.

Ukraine is determined to retake all of its land. From Kyiv’s perspective, repossession of all “temporarily occupied territories” is a condition for negotiations to begin.

Hard nut to crack

So Crimea is crucial for both sides – and accordingly presents critical dilemmas. First, there are the military challenges. As invaders have discovered over the centuries – not least the British and French during the Crimean War, but most notably the Germans who occupied and were then driven off in the second world war – Crimea is a very tough military nut to crack.

In the absence of a large naval and amphibious force, there is really only one entry point, the 5km-wide Perekop Isthmus, where significant and bloody battles were fought towards the end of the Russian civil war in 1920 and during the German invasion of 1941.

Secondly, an assault on Crimea is fraught with political dangers. First, unlike the rest of the occupied territories in Ukraine, most Russians agree that Crimea – with its majority Russian population – is legitimately a Russian territory. An exiled dissident friend, arrested multiple times by police in Russia for demonstrating against the regime, told me: “Few people care too much about Donbas and Zaporizhzhe and Kherson oblasts, they are obviously Ukrainian. Everyone believes Crimea is Russian.”

This is an almost universally held view, even among Russian opposition figures such as Alexei Navalny. While most Russians could tolerate the loss of Berdiansk or Mariupol on the southeastern coast of Ukraine with the same lack of enthusiasm they displayed towards Kherson, they are likely to unite around a defence of the peninsula. This carries with it the possibility (however remote) of escalation, including the use of low-yield (“tactical’) nuclear weapons.

Ukraine may well take another avenue and decide not to assault the peninsula at all. If we assume that they will break Russian defensive lines somewhere south of Zaporizhzhia city – something that will be more difficult than previous offensive operations (due to recent Russian reinforcement) they could decide to direct operations towards recovering Zaporizhzhia oblast and the ”hero city“ of Mariupol (so-designated due to its valiant defence early in the war) in the Donetsk region.

This would restore the lines as at the February 24 invasion and would represent a clear and unambiguous strategic victory for Kyiv. It would also grant Ukraine a winning hand in any negotiations. Russia’s current main supply routes to Crimea would be cut, a task which would be completed by the final destruction of the Kerch Bridge, which is currently under repair.

A leading commentator on the war, academic and former military officer Mike Martin, told me that: “Conducting a full-scale operation to retake Crimea might provoke a backlash in Russia, but putting it under siege while degrading Russian military assets there would put Putin under huge domestic pressure.” The Ukrainians have surprised us before. This may well be the opportunity to do so again.

The US secretary of defense, Lloyd Austin, said in a recent briefing that: “Crimea is an issue to be thought through and sorted out by the Ukrainian leadership.” This is something of a understatement. How the Ukrainian high command resolves this issue will dictate the course, conduct and outcome of the rest of the war.

Whatever course they choose – barring a catastrophic collapse of Russian forces, which will raise some serious issues of its own, to put it mildly – the war is not likely to end soon.

This article was originally published in the Conversation

Frank Ledwidge is a Senior Lecturer in Military Strategy and Law at University of Portsmouth