SandTrout wrote...
It is hard to say what their particular reasoning behind massing over Europe is, other than the fact that they had to mass above somewhere. If they scattered their fleets, that is far more likely to result in the complete destruction of your forces and an inability to expel an incursion than keeping all your ships within range of covering each other.
Tactically, this is called 'mass', and it is a key factor in deciding who will win a battle. Even if the Reapers initially made contact above Australia or China, the fleet would move, as a whole, to bring its full firepower on the enemy at once, as opposed to loosing all of your battle-groups individually as they move independently to counter the threats. This fleet is most likely at some form of battle readiness as well, and not just siting in dry-dock, so to speak.
The fact that the Reaper fleet is so powerful that there was no defense that could have reasonably stopped them does not make the attempted defense tactically stupid or imply that the commander was an idiot. It only implies that the commander was sorely lacking in intel regarding the threat they were facing, or that they were desperate to protect our home-world, either of which are understandable courses of action.
Actually....
The Alliance doctrine of space warfare, as explained in the codex, is one that makes you wonder if the fleet massed over earth at all.
According to the codex, the alliance makes up for its lesser numbers in fleet compared to other races by abandoning the hope of being able to defend everything. They got token defence forces stationed more to be able to warn about impeeding invasions than actually prevent such. The doctrine then calls for the total fleet to pinpoint hit strategic targets or battles, in order to be able to make up for lack of strength. The alliance fleet is thought to be 'highly mobile' in this regard.
If anything, I'd see Alliance accepting Earth as a lost cause and attempt to make it a honeytrap instead, retreating any ships they have to Arcturus or another gathering point before they can rendevouz with outside forces for a proper 'pinpoint attack', instead of wasting themselves on a fruitless endeavour.
Residents of Earth itself, however, might be of quite a different viewpoint in this regard, though, and more inclined to try an ultimately hopeless struggle. Both because they prob can't get away in time due to short warning and alliance keeping a lid on things in order to prevent panic at 'closure time', and because according to shepard that's just what humans do

(fight against hopeless odds)
I can fully understand a token fleet from earth nations trying what they can, but I would expect the alliance to follow their own doctrine and call earth a lost cause before submitting any forces to that battle they didn't have to (ie. some of their ships might not have gotten the word fast enough that they got a charlie foxtrot at earth and need to get the heck out and therefore get caught in the area when the reapers arrive)