CHAPTER XXXVIII.: feeble republics are irresolute, and know not how to take a decided part; and whenever they do, it is more the result of necessity than of choice. - Niccolo Machiavelli, The Historical, Political, and Diplomatic Writings, vol. 2 (The Prince, Discourses on the First Ten Books of Titus Livius, Thoughts of a Statesman) [1513]
Online Library of Liberty - CHAPTER XXXVIII.: feeble republics are irresolute, and know not how to take a decided part; and whenever they do, it is more the result of necessity than of choice. - The Historical, Political, and Diplomatic Writings, vol.
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Resolute is like firm, determined, willful. This does not necessarily tell us anything about the suitability of the decisions. Could be well reasoned, experienced, informed, cautious, etc. Or they could be rash, ill informed, etc, or some mixture. And so irresolute doesn't tell us anything about insight, experience, etc either.

Decisive is a little complicated, because there are different kinds of decisive. One form means like determinative, like having a direct causal relation. "The battle decided the future of the empire." Another refers to the quality of a persons character.

Some contrived examples:

"The two opposing commanders were resolute, but the resulting battle was indecisive in determining the control of the valuable trade route."

"The two opposing commanders were inexperienced and quite irresolute, but the resulting battle was completely decisive, as one side made fewer errors."

The two concepts irresolute and indecisive are different, but have some overlap, as they seem to feed into one another.