1. After First Contact, wars, disease, hunger, etc. on Earth were pretty much eliminated. Presumably if there's no war between humans, then there's no reason to make weapons, unless there's some other threat. No external threat was ever implied.
Yet Earth is well stocked in weapons of contemporary interstellar standard, both handheld and ship-mounted - from day one of ENT. This would thus seem to imply a series of external threats, either concrete or imagined.
2. The Vulcans held humans back for 100 years technologically.
Yet in the 2150s, Earth has high warp engines, phasers, photon torpedoes, transporters and long range communications and sensing gear. The supposed holding back has had no observable effect of denying Earth of parity with the leading bellicose space powers of the era.
3. Starfleet was formed not too long before "Broken Bow." It does not seem to be an overt military organization that was formed after a war.
Why not? Archer seems to be its first explorer captain, his
Enterprise its first exploration starship. What would the organization have been doing during Archer's youth if not waging war or preparing to do so?
4. Besides the NX class, the only known Starfleet vessel types were the two warp-2 ships first seen in "The Expanse," and the Sarajevo. They were not referred to as warships at any time, nor do they act as warships other than the fact that they carry weapons.
In their introductory episode, both types demonstrate parity with leading Klingon warships of the era - bettering the explorer
Enterprise in that game! Such vessels could easily have fought a successful war or six in the recent past.
(Assuming that they weren't armed with the embarrassing plasma peashooters and missing missiles that the NX-01 left the pier with in the hasty launch depicted in "Broken Bow", that is. Shipboard phasers may be a recent "war-enabling" invention. OTOH, pre-phaser shipboard weapons exist, and they probably do so for a reason.)
5. There is an Earth Cargo Service mostly operated by "space boomers," who occasionally have to deal with space piracy. There's no indication that Starfleet or other former space agencies had ever helped the ECS deal with this problem.
Which might indicate they have better, more pressing combat-related things to do. The nascent United States engaged its forces in conventional naval war against major adversaries, France and Great Britain, before building up its reputation in anti-piracy action.
6. No one ever made any comments about Earth being involved in any wars prior to ENT.
True - no "Dilgar wars" for old Starfleet officers to boast on. Then again, we only ever saw one old Starfleet officer up close, namely Admiral Forrest...
And we have to remember that nobody commented on the Romulan War or the ongoing Klingon conflict in TOS, until somebody did. In TNG, Talarians and Cardassians got the same treatment. It would have been in keeping with tradition if a Season 5 episode of ENT had introduced veterans of the old Nausicaan War, having a brawl with the veterans of the 3rd Kzinti War over which of the conflicts outshone the sissy old Romulan War more.
1. Based on the crew's reactions in the first season of ENT, mankind has had little to no contact with alien life other than the Vulcans, members from the medical exchange program, or space pirates.
This would jibe well with Romulan War veterans having had no contact with Romulans...
2. Based on the primitiveness of Starfleet's warp-2 vessels and the lack of true space exploration until the NX program, humans probably haven't ventured much outside their own system, except for the boomers.
This doesn't stop the enemy from venturing to them, though. (An armed response does.)
Based on this, I strongly doubt there was a previous war with an unknown alien race.
OTOH, if we want to treat ENT as being part of the greater Trek continuity, we do have a reason to believe in a series of preceding conflicts - with the Kzinti.
Timo Saloniemi