Hostilities have simmered for years but tensions are now escalating amid fears of a Russian invasion.

Kyiv, Ukraine – According to Washington, Russia has amassed more than 100,000 Russian soldiers on the border with Ukraine and in annexed Crimea in recent weeks.

This has stoked fears in Kyiv and the West that the Kremlin may start a new war with its neighbour and former province that chose to break away from Moscow’s political orbit.

Earlier this month, a top Ukrainian military expert told Al Jazeera that Russia could invade Ukraine as early as January, unleashing a “brief and victorious” war. But Russia denies it is planning an invasion. Moscow says it can move Russian troops wherever it wants and that any of its acts are defensive. Russian officials, including President Vladimir Putin, have for their part warned NATO against expanding eastwards. So, what is at the heart of the conflict that has been going on for more than seven years?

What is now Ukraine, Russia and neighbouring Belarus were born on the banks of the Dnieper River, almost 1,200 years ago in Kievan Rus, a medieval superpower that included a huge chunk of Eastern Europe. But Russians and Ukrainians parted ways linguistically, historically and, most importantly, politically.

Putin has, however, claimed repeatedly that Russians and Ukrainians are “one people”, part of the “Russian civilisation” that also includes neighbouring Belarus. Ukrainians reject his claims. Ukraine went through two revolutions in 2005 and 2014, both times rejecting Russia’s supremacy and seeking a path to join the European Union and NATO. Putin is particularly enraged by the prospect of NATO bases next to his borders and says Ukraine joining the US-led transatlantic alliance would mark the crossing of a red line.

Hundreds of Belarusian exiles gathered in front of the country's embassy in Ukraine on Tuesday evening to honour activist Vitali Shishov, who was found dead in Kyiv in the morning.

Shishov ran the Belarusian House in Ukraine, a group helping Belarusians fleeing persecution.

The Belarusian activist went for his usual run on Monday, but he never returned. Friends were searching for him through the night. He was found hanged on Tuesday morning in a forest in the easternmost part of Kyiv.

Police have opened an investigation and are working on two theories, suicide or "murder disguised as suicide".

But the Belarusian community in Kyiv is convinced he was killed by people connected to Belarus' president. Alexander Lukashenko was reelected last August but critics say the vote was rigged. The election sparked widespread protests and a brutal crackdown on dissent, sparking some Belarusians to flee to other parts of Europe, including Lithuania, Poland and Ukraine.