Ukraine and Poland are neighbours and close allies in today’s conflict with Russia. But the ghosts of victims of an earlier war have returned to divide them. Tens of thousands of Poles were murdered by Ukrainians in Volhynia, in what's now western Ukraine, in 1943. Most of the victims still lie in unmarked graves, and Ukraine has only just lifted a ban on exhuming the bodies.

That followed heavy diplomatic pressure by Poland, about to take over the presidency of the European Union. It threatened to block moves towards Ukrainian integration with the EU unless the ban were lifted.

But Poland’s demand has stirred a controversy inside Ukraine about one of the darkest periods of its history. Ukrainian nationalists who were involved in the massacre - and their leader Stepan Bandera - are regarded by many Ukrainians as heroes.

Reporter Tim Whewell travels through Poland and western Ukraine to try to find out what really happened in 1943, and ask whether Poland and Ukraine can ever lay a fiercely-contested history to rest. And can the record of Ukraine's Second World War nationalists be openly discussed without giving a propaganda victory to Russia, which has tried to use the subject to vilify Ukraine?