Original post to which I am replying is
These are USian-centric questions, but where possible I'll attempt to extend them to include Canada and other democracy-style countries as well.

1. Why does the United States need a large standing military (as opposed to, say, individual state militias)?
Need? For the historical reasons, see: American Civil War, The (1861-65). Currently, if one is to have a military for use outside of one's country (and the US historically has little use for military action inside of its own borders, the Civil War and actions against Natives notwithstanding), it makes sense if it's centrally organised. Given that, the logical thing to do is to make it a federal army.
Individual militias are, I believe, normally under the control of the state governors. Assuming one abolishes the federal army and goes with separate state militias, how can one control them centrally? I believe that the current US constitution allows for the militias to be federalised under certain circumstances (correct me if I'm wrong), but what happens if the states refuse the call? Again, look to 1861.
2. Are "efficiency" or "effectiveness" good enough reasons for our military to develop an internal culture of unquestioning order-following?
No. An army that relies strictly on following orders is quite the opposite, in fact. It is grossly inefficient and not very effective, unless there are large numbers of troops involved (again, historical witness: the Soviet Union, 1942). The Canadian Forces say quite explicitly at all levels of training that yes, one is expected to follow orders, but one is also expected to use one's brain. Illegal orders are expected to be questioned, and reported. Immoral orders are a bit more of a shady area. Morals aren't easily quantifiable, so this should not be surprising. Most Western armies, the US one included, have similar concepts. This isn't really a new concept, it dates back over a century. Historical witness: early German armies.
3. Why shouldn't individual soldiers be held responsible for participating in an unjust war? (for purposes of discussion, let's say we invaded Canada to secure the free flow of Caribou meat and Celine Dion records, and to eliminate the festering problem of Languages of Unbearable Pomposity such as French)
First off, if one is asking a serious question for the purposes of discussion, one should use a serious example.
I'll answer the general question. Why stop at prosecuting individual soldiers? Why not prosecute the politicians who sent them off in the first place? How about the societies that elected those politicians? Democracies are, after all, governments by the people (at least in theory). In a democracy, if an army is sent off to war, the society as a whole has implicitly agreed that the war is just.
There's a potential rebuttal to my answer: "but I didn't vote for that person". Too bad. Leave the country then. Democracies have open borders. Don't like being held accountable for your country's actions? Move to a non-democracy. You can't have your cake and eat it too. Socrates had a few things to say about issues like this. Not enough time between the election and the war, so you didn't have enough time to leave? The soldiers didn't have enough time to quit the military then, either. Can't get or find a job in other countries? Hey, soldiers have to eat too. Try finding a civvy job when your skillset is digging holes in the ground (and then living in them) and the care and feeding of various small arms and vehicle mounted weapons.
4. How many civilian casualties are acceptable, and why?
The short answer to this question is: none. It should never be acceptable that innocents die as a result of a military action. (I deliberately shied away from the usual usage of "unarmed innocents". It's pretty difficult to be innocent if you're toting around an AK-47 in a war zone. That's probably my Canadian bias though, we're not strong believers in owning personal firearms for the purpose of "self defence".)
The long answer is that while morally unacceptable, it does sometimes happen that civilians are injured or killed from military action. Sometimes even friendly civilians are casualties. It sucks. The only other choice is to not take any military actions at all, at which point you may as well disband your armies.
5. ONE YEAR? And that's the MAXIMUM SENTENCE??? WTF???
(Speaking of the conviction of Spc. Jeremy Sivits.)
First of all, Sivits got more than one year in prison. He got a dishonourable discharge (try getting a government job with that on your record, or one requiring bonding, or...)
He also got busted in rank, and that prison term will be served in a US Army prison. It's said that a year in the infantry ages one as much as 5 years in a non-combat trade (or most civilian jobs). I'm sure a year in Army prison ages one as much as 5 years in a civilian prison. I've never been in one, but I've heard stories and they aren't pretty. Furthermore, this is the maximum penalty. Argue that it's too low, if you like, but that's all they could hit him with so that's all he can get.
Furthermore, he gave up information on other troops that will potentially net them more than a year if they're convicted; they'll also face the same dishonourable discharges and demotions. We're used to plea-bargains in the civilian world. We may not like them, but they're useful tools. He didn't get much of a bargain.
A demotion in rank isn't just adding insult to injury, either. I don't know if a dishonourable discharge affects one's pensions benefits in the US Army; I suspect they can't be revoked altogether, but I bet they can't. I also bet one's final rank has an effect on one's actual payout.
In addition to all that, he was mostly convicted, apparently, for things that he *didn't* do. He took a few pictures and didn't say anything to his highers-up. That's not a bad penalty at all. Hopefully this is a harbinger of things to come in these trials, and hopefully the US will have learned from the Canadian example set in the trials of various Airborne troopers in the Somalia affair.