The Kerch strait confrontation is a stark reminder of unfinished business between Russia and Ukraine. Tensions have been rising in recent months, caused by tit-for-tat sanctions, upcoming elections, religious rivalries, unchecked human rights abuses and unexplained “terrorist” bombings.

But the basic problem remains unchanged: the crisis in relations caused by Russia’s illegal annexation of the Crimean peninsula in 2014 after Ukraine’s pro-western Euromaidan protests, the subsequent low-intensity war in the eastern Donbass region bordering Russia, and the international community’s chronic failure to find a solution.

Speaking three days before the naval clash on Sunday, Stepan Poltorak, Ukraine’s defence minister, said the Donbass conflict was entering an “active phase”, with the threat of open aggression by Russia. Some in Kiev suspect Moscow of seeking to impose a de facto sea border, blocking access to the Sea of Azov and the contested Mariupol industrial region.

Russia denies hostile intent, claiming it seized three Ukrainian naval vessels only after they illegally entered its territorial waters and carried out “provocative actions”. The denial is in line with Moscow’s overall position that anti-Russian forces in Kiev are determined to prevent a peaceful settlement.

The two sides are engaged in an escalating sanctions war. Three weeks ago, Dmitry Medvedev, Russia’s prime minister, issued a decree freezing the assets of 68 businesses and 322 individuals, including senior members of Ukraine’s government and the president’s son. The decree was intended to “counter Ukraine’s unfriendly activities towards Russian citizens and entities”, he said. Kiev enacted similar measures last summer.

The political situation is contributing to the instability. Presidential elections are due in Ukraine by March 2019 and the incumbent, Petro Poroshenko, faces a tough re-election battle. Polls suggest Yulia Tymoshenko, a former prime minister, is the favourite to succeed him. Opponents believe Poroshenko could be tempted to play the security card to bolster his chances.

Senior clerics in Ukraine’s Orthodox church are meanwhile locked in a bitter struggle to break the grip of the Russian patriarchate, to which they have answered since 1686. The row, which has caused a historic schism between Moscow and Bartholomew I, archbishop of Constantinople and leader of the Eastern Orthodox church, has inflamed fierce passions among the Ukrainian faithful.

The conflict in and around the Donbass region, between Ukrainian forces and Russian-backed militias and mercenaries, has never wholly ended despite numerous ceasefire attempts. In 2017, one Ukrainian soldier was reportedly killed every three days. In total, more than 10,000 people have died since 2014.

Sporadic clashes continue, plus the occasional assassination. In August, Alexander Zakharchenko, a prominent separatist leader, died in a cafe bombing in Donetsk, which Russia blamed on Ukraine. But equally, internal rebel rivalries or an order from Moscow could have been the cause.

Crimea is held under tight Russian control. Moscow has cracked down hard on pro-Kiev activists, Muslim Tatars, civic groups, bloggers and independent journalists, and is responsible for torture, disappearances and detaining political prisoners, the UN and human rights groups say.

Pressure on Russia and Ukraine to end the conflict has come from two main directions: the US and Germany. Donald Trump has controversially suggested American and EU sanctions imposed on Moscow in 2014 could be eased, thereby implying recognition of Crimea’s annexation. But in practice, his administration, Britain and Nato continue to support Kiev. In July, the Pentagon announced $200m (£156m) in military-related aid, taking total US assistance since 2014 to $1bn.

Angela Merkel, Germany’s chancellor, representing the EU, helped engineer the Minsk accords, which created a process for ending the conflict. But that process has stalled. When Merkel met Vladimir Putin, Russia’s president, in Berlin in August, no progress was reported. Critics say Germany’s more pressing interest is in the Nord Stream 2 pipeline, which will deliver cheap Russian gas via the Baltic.