I know it was "Thread Drift" but Vern's history was wonderful.
Not to step on any toes let me say that there is critical and important.
If you are stringing 5 different 12 foot lengths of rope together to span a gap a little over 50 feet, and one rope breaks, you are screwed. If a guy comes by with a new, very strong 20 foot length of rope, he is critical in the success...but the other ropes are important too.
Same can be said of getting Diptheria to snowlocked Nome, Alaska along the Iditarod trail (and the well known race founded on this historical event, even if the race now overshadows history)
Of course, the serum was moved by train from Seattle to Fairbanks before the harrowing dogsled relay...and without that railway track the serum would never have arrived...yet still it is the dogsled relay that was critical to the juncture,
The fresh US troops with fire in their belly were the critical ingredient thrown into the conflict that changed it from a loss to a victory...but the fresh US troops in a vaccuum would no more have succeeded in winning that the dogsledders of the Iditarod would have succeeded had the medicine not been at Fairbank for them to sled off with.