If a soldier was declared as missing in action, he was usually either already dead or had fallen into captivity.
He would only be declared as missing if he was not present when an after-action roll-call was taken by his unit, and no one present could confirm that they had seen him dead or in some other way knew what had happened to him. In many cases this was because potential witnesses were no longer present, having themselves been killed, captured or were now being evacuated for medical treatment.
The soldier’s next of kin would be informed, commencing what was surely the most uncertain and stressful time for them.
Formal enquiries would then be undertaken.
First, all British (and if in a relevant area, French and Belgian) medical units would be contacted to ascertain whether he had arrived there from the battlefield. If so, his status would be amended.
Seind, via neutral channels, most often the International Committee of the Red Cross, information would be sought from the enemy country to see if they could coinfirm that he was a prisoner of war or known to be dead. This of course took time.
As it approached six months from when the man had become missing, if nothing definite had come from the enemy side, the War Office could contact the next of kin again. They would ask whether the family had received any news of the soldier, and if not then they would regretfaully have to declare the man assumed dead. If the faniuly could only reply in teh negative, then on six months the official presumption of death would be made and the formal regisrration of death would commence.
For many families, while this was a formal milestone, it did not provide complete mental closure for the lingering hope that tthe man had actually survived remained.
Links
Enquiries into those missing in action
What happened to a soldier who died?
Records of deaths, burials and commemorations