I see what you mean, but this is a figure of speech. The first paragraph is a description of what is happening, almost documentary-style. This happens, then they learn about this, then they spend some time on heavy machine guns.
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Ivanhr"This" is used correctly in your example. "This" is the most obvious choice when you want to give more emphasis to a new situation, event, subject etc. While all "it", "this" and "that" can be used to refer back to what's already been said, "this" and "that" are simply more emphatic than "it". According to Swan, "this" is also preferred when the speaker has more to