Which preposition to use with fraternisation

of Occurrences 2%

The American officer complained that after a recent fraternisation of the two forces which had taken place in accordance with previous arrangements near the "wood mill," on the departure of the Red troops he received reports that the Red Guard officer had ordered the destruction of certain machinery at the mill, and had also torn up two sections of the line at points east and west of the station at Svagena.

Which preposition to use with  fraternisation