In simple terms, the question is really easy, and there are plenty of examples from earlier times that could be cited. However, like most everything else in life, simple questions don't have simple answers, especially simple right answers.
Around the world we are watching humanitarian disasters in the making.
Let me first dispense with Japan, because there is little the Japanese did that they can be faulted for. Mother Nature can always summon more power than man can overcome, so unless you believe this was some sort of cosmic karma payback for something, this is, unfortunately, just something that happens. If any country was prepared for it, Japan was.
Later, there will be a time to examine the issues with the nuclear plants, and certainly there will be lessons to learn there, but right now it's time to help those who survived, and, where possible, find and identify those who didn't. There will be too many of the latter, and, realistically, too many of the former too, for this thing was just off the scale.
However, on the other side of the world there are similar tragedies occurring, and the loss of life will be similar. Libya is still a mess, and will be for some time, regardless of the eventual outcome. Now Bahrain is making news again, and a new dynamic has been added, with the government openly calling in troops from other countries.
Why? In simple terms, it's hard to get soldiers to fire upon their own fellow citizens. It's always been true, and the easiest example is to look at the Berlin Wall. East Germany had to bring in troops from the outlying areas because the Vopos (People's Police) from Berlin were hesitant to fire on fellow Berliners. They understood the desire to escape, even if they didn't try themselves. So, the guards were brought in from far away, and were much more likely to follow orders and shoot to kill.
However, the simple question I alluded to earlier is this: At what point does the world intervene in the internal affairs of a country to prevent a humanitarian disaster?
No one would argue that had we stopped Hitler, things would have been much different. His actions, while perhaps a little more subtle, were known and tolerated well before the shooting war started in 1939. There wasn't an open resistance or protesters screaming for help, but...the world knew a great deal. In hindsight, the answer is very clear, but hindsight is always that way.
How many protesters must die before the various despots are challenged? Is there a magic number?
When is the magic time right? For the protesters, clearly it's before they're all dead, but how do we determine that?
Russia and China hold the keys, I think. They have veto power in the UN. Unfortunately, they also have one other thing to consider. Both are repressive regimes, with significant numbers of people who will/might/would rise in protest themselves. The Russians know this well. While they might agree to intervention in Libya, they fear that precedent should their own population revolt, hence they don't want to start a trend.
Russia needs to think a little deeper. They still talk about the great damage done when Hitler invaded the Motherland. They could have acted, but didn't, and paid the price. Is today any different? Does it matter that it's Arabs killing Arabs and not Aryans killing Jews?