I was thinking recently (considering ideas for the continuation of the Enterprise RPG [/shameless plug]) about the question of fleets in star trek. We know by the time of the Dominion War and DS9 that the Federation has numerous fleets numbering probably at least 100 ships each, but in TNG having 40 ships together was the best Starfleet could do (BoBW). And yet it seems like the loss of 40 ships was relatively light ("We'll have the fleet back up in less than a year."). This makes me wonder, is it possible that the fleets as we saw them in DS9 always existed but weren't fully mobilized until the approach of the war. The amount of planning and resources required for the Federation to mobilize a fleet to attack Dominion shipyards and then attempt to retake Deep Space Nine right at the outset could not have been completed overnight so I presume that, despite whatever we may think of TNG-era Starfleet someone, somewhere was planning for a war with the Dominion. Based on this I thought up my own theory on where the fleets come from:
The fleets have always existed at an administrative level. In Starfleets organizational charts (or whatever), there is a list of total ships assigned to Fleet X, but they are not all mobilized at the same time. For most of TNG the Federation was at peace. Yes there was the ongoing "war" with the Cardassians but by the time of TNG it seemed to have been reduced to the occasional skirmish. This level of conflict would (likely) not require mass fleets of starships. It is also established in TNG that it is rare for some vessels to meet each other in space (I think Picard mentioned that regarding their meeting with the Yamato) and/or for some starships to return to earth (I believe that was mentioned in Conspiracy). This makes me consider that during peacetime, Starfleet operates fewer ships, and more of them on long-term, long-distance missions.
We also know that Starfleet apparently has some sort of personnel reserve in effect ("You're revered Admiral Nogura invoked a little-known, seldom-used 'Reserve Activation Clause'!"), and it would be sensible that they have some sort of Mothball/Reserve ships somewhere (wasn't Qualor II a surplus-depot?). Possibly these reserve ships and personnel are continuously assigned to a particular fleet, but only called-up in wartime so its simply a matter of calling back the crew and readying the ship to depart to a rendezvous point -- admittedly this may take several months but certainly Starfleet had several months to gauge the potential for conflict with the Dominion.
I think use of reserve personnel and ships may go a long way to explain the discrepency in fleet numbers we see in Star Trek. In TNG 40 ships was a lot, and Picard had trouble finding enough ships for his Tachyon Net IIRC. But in DS9 we routinely see fleets number ~100 and then again in Nemesis Picard says "Our fleet has been diverted to meet us there," yet the graphic does not display anything close to the amount of ships we see per fleet in DS9. Possibly the display shows only the ships that arrived at Sector 1045 but its also possible that now, so many years after the DW that Starfleet is settling back down into a peacetime routine and the ships displayed are merely the operational ships attached to the fleet the E-E belongs to.
Does this make any sense? What do we think?
"All this has happened before --"
"But it doesn't have to happen again. Not if we make up our minds to change. Take a different path. Right here, right now."