However, they have to be pretty committed to this, because if someone outside of the group does eventually mine a block that contains the censored transaction, the group members also have to be willing not to build on that block, and to trust each other not to do so. Just agreeing not to include the transaction in blocks themselves would be insufficient.
At that point there are complicated questions about how stable and credible the arrangement will be, since systematically avoiding building on certain mined blocks involves slowing down the consensus slightly, and also trusting that the eventual consensus will exclude those blocks.
If, say, 51% of mining power committed to censoring one particular transaction, that might yield an effective fork of Bitcoin for some period of time because the other 49% might gamble on their ability to get their rival blocks accepted in the long term (or, if they didn't know about the 51%'s policy, not see any reason why they shouldn't simply build on what appeared to be the longest chain!).
A pithier version would be: it's infeasible to censor transactions and if a nation state spent enough to try it, it would indeed be noticed and the community would fork.
Look at the Ethereum Classic debacle - and remember that many Bitcoin users tend to hold those views stronger than many Ethereum users.
PS: Add up the Chinese pools and one country effectively already controls bitcoin.
Looking at the chart, you can see that the top 5 mining pools can collude in being a single entity to "control" the blockchain (I believe this is called a Sybil attack [https://en.wikipedia.org/wiki/Sybil_attack]). Now keeping incentives in mind, the idea is that people using bitcoin have faith in the system. That gives bitcoins value. Miners get paid in bitcoin. If they were to manipulate the blockchain, that would most likely cause bitcoin's userbase to lose faith in the system and thus what the miners would be stealing would suddenly have little to no value.
I'm not positive, but I recall hearing that pools have been intentionally been made smaller to keep users' faith in the system. Perhaps the goal of these majority mining power is not to steal bitcoin but censor certain users? That may be a bit more complicated depending on the users' opinions on a case-by-case basis. Fun stuff :)
A Sybil attack can occur when a system designer mistakes a system identity to be exclusive with other ones. e.g. you could create a website that used a phone number as a user identifier and give every new user a $5 signup bonus. But if someone found a way to create valid phone numbers for less than $5 each, they could exploit your new user bonus system.
If it's free/cheap/easy to create new identities, you must be careful how much weight to give those identities. e.g. reddit has some problems related to how easy it is to manipulate voting. They have probably made changes to their algorithm over the years to deweight new/unverified users and likely other clever things to detect voting rings.
Big miners can easily delay your transaction, by refusing to include it in a block, but the only way they can prevent your transaction entirely is if all of the miners collude not to include it.
It's still effectively impossible because it would undermine confidence in the system (which is why people are willing to pay miners in the first place).