Can Russia make a truce with Japan (without USA/UK knowing)?
It looks as if your friend has read too much history and not enough of the rulebook.  Russia did have such a truce with the Japanese (actually, there was no declaration of war until Germany had been beaten), and I'm sure Stalin would have loved it if the US and Japan had pounded each other into shreds while the Red Army was busy.  But Stalin, whatever else he may have been, was a good game player: the troops he transferred from Siberia to Moscow kept him in the game that turn, and allowed him to win on one front while keeping the other at status quo till the last turn, when he made a dash for the Far Eastern victory points.

Getting back to the question: of course secret inter-player agreements are 'allowed', if only because there's no way of stopping them.  But they only work if both players believe it is in their interest to keep to the agreement.  In A&amp;A, it's a zero-sum problem; this Russian strategy benefits either the Axis or the Allies, and all the information to work it out is available to everybody. In your game it was a very bad plan; but in Stalin's game it worked like a charm.

A friend of mine summed up a similar situation as "Everybody's happy? Then somebody's miscalculated."