The Romulans had been in isolation for decades, perhaps even the better part of a century. Then they show up with ships over twice the size of the Federation's best and biggest, cloak equipped at that, and powered by a new type of power generation of great potential. Couple that with the traditional Romulan strategy of sneak and strike, and I'd be afraid of going to war with them myself if I were the Federation. As Picard said once while reviewing sensor data from a sector of the Neutral Zone "There could be an entire fleet out there and we can't even see them". The old shibboleth about "no warp Romulans" is a misinterpretation of Scotty's line about "simple impulse", which referred to the ship's power source, not a lack of FTL capability. Impulse power is basically fusion power. All of the statements in Balance of Terror where the Romulans are obsessing about power, about fuel, about "reserves" makes perfect sense if fusion power is powering their engines as well as their "big gun", the plasma-implosion torpedo.