Which preposition to use with interpenetration

of Occurrences 14%

But a common victory, and a common ardour in rebuilding the waste places, and binding up the broken-hearted: even they will not be enough, unless, beyond the war, all three nations, nay, all the Allies, do not set themselves to a systematic interpenetration of life and thought, morally, socially, commercially.

Which preposition to use with  interpenetration