I don't think so: for example the Weimar Republic is a counterexample. Or a modern example is the current war between Ukraine and Russia (both are democracies even though in Russia some particular president and party has a lot of influence). Both of these examples show that even in a democratic country under specific circumstances it can happen that a particular group or party can gain lots of power.
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If you want to see another interesting "peace theory", consider the madman theory:
> https://en.wikipedia.org/wiki/Madman_theory
It basically says that if a country has access to atomic weapons, and the president or person in power radiates the image that he is sufficiently "mad" that in case of an aggression against the country they are willing to use them and escalate the conflict into a global nuclear holocaust, other stakeholders will strongly attempt not to upset this country, i.e. seek more peaceful and diplomatic solutions with this country.
This theory explains well why the USA have not yet invaded North Korea and are hesitant to invade Iran.
Most modern wars don't make financial sense when accounting for everything but they are still fought. Often over stupid reasons.
For example Russia invaded Ukraine despite trade partnership with Ukraine and EU.
https://en.wikipedia.org/wiki/Democratic_peace_theory has a better run