Starfleet is basically always at peace, even when it's not. That's probably why they have such a hard time mobilizing a coherent defense when Big Bads show up on their doorstep unexpectedly.The one variable wrt Pearl Harbor and the like is that the US was at peace that day. Is Starfleet ever at peace?
Also, I believe the "fleet in being" concept is nonsensical in this concept since Starfleet's most powerful ships are almost always designed to act as long-range exploration platforms. By analogy: if the U.S. Navy had that kind of exploration initiative in 1941, all of their "battleships" would have been away exploring the antarctic or conducting geographical surveys of the ocean floor when the Japanese attacked, and Pearl Harbor would have been filled with submarines and destroyer escorts that hadn't built for long-term endurance and therefore had to put in to port between patrols.
Can it counter even devious Klingon plans, let alone those of the resurgent Romulans, if its battleships (Federation class dreadnoughts, whatever) remain idled at Pearl Starbase?
Starfleet officers are a group of warrior-scientists who depend more on cleverness and technology than firepower or numbers. In the event of Romulan resurgence or Klingon deviousness, their immediate response is to send a shipload of their smartest officers to sneak up to the enemy's supreme commander and tie his shoelaces together.