The Helsinki accord was a masterpiece of European diplomacy. Fifty years on, we need its spirit more than ever | Kai Hebel and Richard Davy

The OSCE hasn't stopped Putin's aggression, but it has the potential to help broker a peace in Ukraine when the time is right

Vladimir Putin will probably never give up on his attempts to bring Ukraine into Russia - which is where it belongs, according to his warped view of history. Those who oppose him tend to fall out of windows or suffer other accidents" or go to prison.

If he agrees to a ceasefire, it will be only to gain time to replenish his forces before trying again. All that would stop him then would be armed peacekeepers of some kind, as is already being discussed. If someone replaces him from his inner circle, there is unlikely to be change.

Kai Hebel is author of Britain, Detente, and the Helsinki CSCE (Conference on Security and Cooperation in Europe); he is assistant professor of international relations at Leiden University. Richard Davy is the author of Defrosting the Cold War and Beyond