The Republic emerged from the rebellion stronger and more solid than ever. It's been the presidents since - Theodore Roosevelt, Woodrow Wilson, FDR, LBJ, Nixon, et.al. - who went and threw all that away.
Strength and Power are not the measure of a successful Republic. Consent of the Governed is the measure of success for a Nation formed on such a concept.
Hitler's Europe would have been a strong Union and probably more solid than any other, but it would not have been a successful Union according to the foundational principles of this nation because it would not be ruled by the consent of the governed.
The Soviet Union was another nation that ruled against the consent of the governed, and no doubt by your assessment, it would be regarded as solid and strong.
That misses the point completely. Your argument simplifies (in the mathematical sense) to "Might makes right."