My interpretation of the german term :
>> ... sich gegenseitig
wieder flottmachen ... <<
would be, that indeed some of the of the tanks got bogged down in the swampy terrain.
While achieving to get those free, some of the helping tanks got bogged down, too.
Those could finally got going again by the help of some of the tanks, that had been freed before.
So an alternative translation could be :
...a number of tanks could only free themselves [from the swampy terrain] by mutual assistance.
@l3w : I hope that's not too
nitpicking, as the final result is not different from your translation (or Hiltrauds),
but I thought it might give a little more of an idea, what was going on that night.