When a horde settles down it is only the units that have 'horde' in their title that will disband as these are your people pressed into military service. When given the chance to have a settled life again, they take it.
Other military units will not disband. They are your 'regular' military forces that you have before becoming a horde. They will remain after you stop being a horde.
If you have taken a lot of casualties among your regular (original) military forces, then you will notice that your total combat power drops significantly when you settle.
If you have taken casualties among your horde units, when you settle you'll find it harder to get your new settlement growing and/or recruit new military units.
Hope this clears matters up.
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Gentlemen should exercise caution and wear stout-sided boots when using the Fintry-Kyle Escape Apparatus. Ladies, children, servants and those of a nervous disposition should be strongly encouraged to seek other means of hurried egress.