The president is the supreme chief when it comes to the military and what the president decides the military has to do. Of course, they have numerous advisers who help them so it's not like they go and plan worldwide missions as they see fit. The president also negotiates treaties with other countries and represents the country in any affairs with foreign countries, both enemies and allies.
The congress has to ratify these treaties or give their approval for leading a war. This is to prevent the president from doing whatever they want to do and from being tyrannical and leading to pointless wars or getting treaties that don't benefit the American people. However, due to certain loopholes and various acts, there have been historical wars that presidents organized without the support of the congress.
The division of power should be considered reasonable because nobody can decide things on their own.
The Second Triumvirate was officially recognized by the Roman government in 43 BC. Some historians consider this to be the end of the Roman Republic. The Second Triumvirate ruled for ten years until 33 BC. However, it began to split up when Octavian removed Lepidus from power in 36 BC.
Justinian I – Emperor who ruled the Eastern Ro- man Empire, or Byzantine Empire, from 527–565 ce and ordered all Roman law compiled into a multi-part work referred to as the compilation of Justinian and later named the Corpus iuris civilis.
