What would be the psychology I wonder behind depriving men of leave rights at the last minute?
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It's a simple psychology exercise to see if you can 'hack' it.
Imagine being in the field, you're due a few days R&R when suddenly, the situation changes within your theatre of operations. You are required to stay, forego your leave and soldier on because the situation dictates it.
Do you soldier on, or as the Royal Marines say 'dig out blind', or do you become sullen, have a fit of pique and leave your colleagues in the lurch with your resentful attitude?
It's classic, basic training stuff and is meant to test your mettle, your resolve and your attitude. It sorts the men from the boys and those who have the character come through.