Let's check the fictional history of Trek.
2311 - Treaty of Algeron
2379 - Contrary to the warmongering notion that Romulans are devils who would never ever want peace (why then did they sign treaties with the Feds in the first place?) the events of NEM imply the chance for a real peace. Of course we are not naive, we all know that unlimited expansion is THE Romulan dogma. But even the most rigid Vulcanoid societies can change.
Let us analyze Nemesis:
Enter Shinzon, a human - hated by romulans. He represents the remans, hated by romulans.
And, yet, the romulans - the romulan military, to be exact: the power any praetor has to have behind him in order to have a life expectancy longer than a few days - overcome their utter disgust to such a degree that not only are they (along with other romulans) complicit in the utter elimination of the senate, but they give Shinzon the leadership of the romulan empire
And what caused the romulans to overcome their deep seated racism and disgust to such a degree?
The fact that Shinzon had a weapon that could kill everyone on Earth and the fact that he promised the romulans to use it in order to sterilize Earth, killing billions.
Feel the love.
And this behavior of the romulans was the same from TOS to TNG to the last 24th century canon.
Of course we are not naive
Hitler pre-1938 was WAY less aggressive towards the allies than the romulans are towards the federation.
I mention this because the appeasers of that period also name-called the ones that were not foolishly naive as warmongers. And they were also hailed as heroes - until the war came, at least. A war made FAR bloodier by the actions of said appeasers.
So yeah, looks like the few decades prediction has not come true which is hardly surprising. (...)Add the alliance between the Feds and the Klingons and it becomes obvious that the Romulans will not seek war as long as a rough balance of powers is maintained.
Helping the Duras family, in order to break the federation-klingon allience.
An attempted attack on Vulcan.
And I am not even counting:
A plot to capture Enterprise, a high-profile starfleet capital ship - by using a defector carrying false information. A clear act of war.
Attempting to destroy Enterprise that just helped the romulan ship. A clear act of war.
Of course the romulans do not try to start wars as long as the klingon-federation alliance/rough balance of powers/whatever exists.
The only reason a federation-romulan war has not happened until Nemesis is because the writers were not self-consistent:
A war was not depicted (only a very bitter cold war) - maybe they did not have the budget for a war, or the war would have been too serialized for TNG, or they had a lot of other plot lines on their hands, etc.
Why is that not self-consistent? Because the way the romulans were painted, war is inevitable.