Does separation of political powers curtail us from the imperialisms of the past states? Or is a further separation of powers necessary?

Reading the Wikipedia article on separation of powers, I take notice of the other branches of government that have been added to the systems of government in a few countries:

• In both Costa Rica and Venezuela, the electoral and auditory branches
• in Germany, the constitutional court and presidential electoral college
• in Taiwan, the control (auditory) and civil examination branches

The goal of the separation of powers is to keep the direction of government policy from falling in the hands of a single individual (dictatorship or totalitarianism) or group (oligarchy).

But that separation of powers did not prevent the Roman Republic (separated between the senate, the executive and the Roman assemblies) from turning into the Roman Empire, nor did it prevent the early United States from moving ever-so-westward to California, stomping other, “lesser peoples” into either the ground or the reservations along the way, resulting in a U.S. empire that stretched “from sea to shining sea” (and then some more land on the other side of that other sea as well).

Yet, let’s face it. Both the Roman Republic and the United States were/are excessively dependent upon their militaries as defense mechanisms, even though the United States, since the end of WW2, uses its own military (and its accompanying goods) more as a “big switch” to shake at errant nations.

So what does it take for us to extricate the government from the use of armed force as a means of accomplishing foreign policy aspirations?

Should the separation of powers be further spread out among a greater number of branches, which can then be made much more accountable/responsive to the citizenry by periodic ballot?

Will such a further delegation of powers place a plug into the need for a military that mostly resides outside the government’s grasp?