Summary

Yosef Karo Shulchan Aruch — Yoreh Deah 375

If they sent their deceased to another city in which case the law is to start mourning when they turn around, and they began to count the period of mourning, and afterwards, the deceased was captured and it was not possible to be bury him for a long time, they do not need to stop their mourning and they do not need to go back and mourn afterwards, for since their rule is to begin immediately, they do not need to stop.
Source: Wikisource

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