Russian forces have launched a major military attack on Ukraine, on the orders of Russian president Vladimir Putin. Tanks and troops have poured into Ukraine at points along its eastern, southern and northern borders, Ukraine says, and explosions have been heard across the country. But why is Ukraine being invaded, and what might Russia want from its neighbouring country? To make sense of the current crisis we must understand the history of the relationship between the two inextricably linked countries, which dates to at least the 9th century. Here, Professor Serhy Yekelchyk, an expert in Ukrainian history and Russian-Ukrainian relations, highlights nine milestone moments…