@wolfwood:
For the comparison to be apt they'd have to wage war against an active Nato member.
Literally murdering the people in their own alliance.
I don't know why you're not including the countries affiliated to the UN, but that doesn't really bother the point I am making, since I'm including "a lot of bad shit" to the countries that the USA were/are allied with (not only open wars)… and overall breaking many principles of international relations that didn't lead to any retaliation.
Also, bringing USA to the discussion as you did is not really a equivalent comparison to the World Goverment, a system much more vertical than horizontal and even more unbalanced in power.
So you want to leave the World Goverment because you don't feel safe around the Tenryuubito anymore after the Shirahoshi incident? Oh, you can do that! There are countries that are not under their rule anymore. Anyway, good luck when the Marines leave your island because it's out of their jurisdiction and when the Gorousei demand that other countries cease their commercial relations with you... oh, and don't expect that every other country to leave with you, because many of them will have reasons to stay with the WG (benefits of the system, corruption, cowardice, etc).
Of course, if you're a ballsy king with some influence over some other countries and start to become enough of a pain in the ass to the WG, the WG will declare war against you and you'll be erased from the earth. Every other country will be scared as hell, but they will silence their nerves out of fear while the newspapers will be telling a very compelling story about how dangerous your country was to the world. A few decades later, their fear will be already gone and everything will be friendly again, because the world will be peaceful to the allies of the World Goverment. The Marines will be the daily heroes of the people, the market will be sweet, everything in balance.
Let's go the Reverie talk about politics.
--- Update From New Post Merge ---
@Miss:
We've now seen two cases where the CD have left Marijoa to enter other sovereign countries and wreak absolute havoc. Ridiculous taxation aside, they've entered other nations to kidnap and/or murder its citizens and if you try to interfere in any way, the people who will stop you will be the Marines. My point was that they're as much apt to help you as they are to stand by and watch you burn. Just because an incident is "isolated" doesn't make it any less damning.
I agree with you that the best outcome would be for the CD to go away and let the Marines operate autonomously, but as it stands right now, they're just as complicit as Cipher Pol.
Two cases? One of them is the Ryugu Kingdom, an isolated country that nobody cares about with a long history of discrimination. The other place is Sabaody, the unnoficial garden of the Tenryuubito (because it's located right below them) and that we don't even know if it's a legitimate country. Oh yea, I guess the people from [insert kingdom] probably hate the Marines because some people were slaved in those other 2 islands. Anyway, it's pretty much established in Doflamingo's flashback that the people hate the Tenryuubito… but we've never seen anybody hold anything against the Marines that protect them in their daily lives.
That said, did you read the rest of my previous post to you? I'll repeat a little part that is there right now:
"The other kinds of evil that the Tenryuubito cause when they abuse of their authority happen in a much smaller scale: slaving stray individuals and not being progressive enough to end the discrimination of minorities, like the fishmen people. All of this is totally terrible, but it's not something that directly affects the welfare of 98% of the affiliated countries, neither affects the politics of the world. Most of the kings probably don't even care about slavery and discrimination. There's also the fact that the press is biased towards the WG, so many of the biggest tragedies caused by the WG were mistold to the world, not being held against them.
Therefore, the cost-benefit analysis of why you should join the WG, despite the Tenryuubito, is not bothered too much by these deviations of morality. All in all, you should also consider that Oda frames the bad much more than the good."