Why did my troops lose to a kingdom with less power?

The only possible reason is that the kingdom that you have attacked (let us put into consideration that you have attacked someone of the same level), might have higher tiered soldiers compared to that of yours. You should also practice scouting the base first -- for it is possible that he/she was reinforced by a player that is far more advance in level than you are. To check, you might want to look at battle reports and figure out what actually defeated your troops.

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