@Marc NobbsIn my WWII story I have reliable records on where particular Divisions are from 1939 to 1945. The component Regiments I can often trace too. Battalions, I have some knowledge, in particular for key times. Companies only in some specific instances. Since the story is told from the perspective of a man at the Platoon/Company level, I need to "fill in the details" conforming to historical events.
I am using contemporary sources, individuals of similar rank and duties, who recall hearing about significant events outside their "bubble" such as the attack upon Pearl Harbor and other facilities in the Hawaiian Islands, and the Philippines on December 7th 1941, or the death of FDR. Events such as the 1939 Worlds Fair (increased availability/marketing of 35mm color film), Train Schedules (available form railroading fanatics), or a flood that effected training; are I think important, as they provide interest beyond the purely military. Also, I am trying to inform people of Who those soldiers were, and things that influenced them.
My modern war stories I keep a calendar of historical events, in particular those that would distract people higher in the chain of command. I maintain separate time lines for the various factions involved (including "friendly" forces) because Logistics and what preparations have been done, or not done, dictate what various units may do.
My stories are grounded both in the doctrine and manuals of the US Army (of various eras), historical experiences, and my service that includes participating in the planning and movement of 500 to 1,000 men or more of an Infantry Battalion, plus attachments, and minus detachments, to fight wildland fires, or floods, as well as for training, and in war. Plans are great, but your opponents have plans too...
Or, as Mike Tyson said, "Everybody's got a plan, until they get punched in the mouth!"
Readers don't see the various timelines, they just see particular events. Some readers may "connect the dots" and reasonably anticipate certain events...
For example: Friendly force A has been ordered to travel 100 km to liberate a town from enemy force X.
However, enemy force Z has been ordered to set up a defense/Ambush against force "C"
Force A has a decent plan, travel by trucks, dismount, conduct a reconnaissance, and attack with a supporting Flanking attack too.
Unfortunately, higher HQ gave priority to force B travelling the same highway, delaying force A. Now, with time running out, the HQ demands force A speed up, and they don't have time to conduct a recon, nor move part of their unit to the flank...
Both sides had decent plans, but the other side is not doing what they expected...