I would say that "I was impressed by the courage of soldiers" is less usual or expected rather than definitely incorrect. I think that the direct personal experience of "I was impressed by" makes this particular group of soldiers seem more specific and therefore deserving of the definite article. " is likely to be "The ones I met".
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GPYI would say that "I was impressed by the courage of soldiers" is less usual or expected rather than definitely incorrect. I think that the direct personal experience of "I was impressed by" makes this particular group of soldiers seem more specific and therefore deserving of the definite article. The answer to "Which soldiers?" is likely to be "The ones I met". On the