I've noted that the Romulans have been a constant problem in the various Star Trek series. I'm just wondering who started it all? and why are they a constant problem? Plus I want to know more why the neutral territory was formed?
who started it all?
In 2152, Humans made first contact with the Romulans when the Enterprise NX-01 encountered a Romulan-laid minefield. Communication was via audio only. The Romulans saw that Humans fostered a spirit of cooperation among the long-belligerent Vulcans, Andorians and Tellarites. Realizing that this would bring solidarity to the region and an obstacle to conquest, the Senate took steps to turn these species against each other.
In 2154, Romulans conspired with V'Las, head of the Vulcan High Command, to invade Andoria. V'Las' Romulan contact had the stated agenda of reunification with the Vulcans.
A few months later, the Empire sent prototype holoships remote-controlled from Romulus to disrupt a peace conference between Andorians and Tellarites. The Romulans piloted the ships using an abducted Aenar; however, their scheme was thwarted by the combined efforts of the Humans, Vulcans, Andorians, and Tellarites, led by the Enterprise. This enraged the Romulans, who vowed revenge upon humanity.
About 2156, the Earth-Romulan War began. Both sides, for unknown reasons, used weapons of an atomic nature. These were still in the Romulan inventory a century later. The war only ended after both sides had fought to the point of exhaustion, and realized that further conflict would result in mutual destruction.
why the neutral territory was formed?
In 2160, the Romulans and the Humans signed a treaty ending the war and establishing a neutral zone one light year wide between their territories. The treaty was negotiated via subspace radio, again with no visual contact.
why are they a constant problem?
Out-of-universe, Romulans were modeled on Roman empire. The way the empire worked economically and politically was by constant conquest.
In-universe, they are an off-shoot of Vulcans who rejected Surak's reforms during the Time of Awakening and therefore were goverened by threir emotions. The society was highly militarized, totalitarian and xenophobic.