The disagreement between Russia and the west over the Conventional Armed Forces in Europe treaty is one of the most complex issues on the diplomatic agenda this autumn. Yet, for all the ­complexity, what is at stake is the survival of a hugely important defence agreement, one which neither Moscow nor Washington can afford to see abandoned.

The CFE pact, signed in 1991 and amended in 1999, not only limits the number of tanks, artillery, military aircraft and helicopters deployed by Russia and Nato states, it also contains a raft of confidence-building measures involving, for example, mutual notification of overflights or the firing of ­missiles.