I've noticed some authors of academic pieces tend to put square brackets around a T when starting a quote, I don't understand why.
"[T]he Organization is based on the principle of the sovereign equality of all its Members."
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This is because the original quote had a lower case "the" rather than "The".
Square brackets are used around words that are not part of the original quote - that is, words that have been inserted or, as in this instance, modified to correct the case.
This typically occurs when the quote needs to adapted slightly to match the grammatical context in which it is placed, e.g. change of tense.