The continuous activity mirrors how the requests will be done when the time comes. In the portion, you can read that Hemingway utilized the expression "when it will begin." The pronoun "it" alludes to a particular occasion that is imperative and should be completed as proficiently as could be expected under the circumstances. The long sentence successions which moves the military will make to set up for the occasion.
I talked with the major and learned that when it should start and our cars should be loaded we would drive them back along the screened road and up to the main road along the ridge where there would be a post and other cars to clear them. What effect does the style of this long sentence achieve?