To be fair, I think it would be fairly easy to kill people in any country with open borders.
Send in the guns, novichok or raicin tipped umbrella with diplomatic baggage. Assign a couple of highly trained people to the assasination, then make sure they are out of the country before it's even clear that there has been some malpractice.
The only question is whether or not the leaders of a specific country want to get involved in such matters. Most do not.
Then there are the exceptions who do get involved, like Mr Gadaffi, Fat Kim and leaders of certain larger countries.
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How do you think a visa would be a barrier to carrying out an assassination in a foreign country ?
I expect the UK could apply for visas in Russia, send in the guns/poison in diplomatic bags and carry out an assassination. The difference is that, in recent decades, we tend not to do that as the UK is not run by the mafia.
The only way to robustly prevent assassinations is to stop people coming in or continuously monitor all foreigners. I expect it would be fairly difficult to assassinate someone in North Korea, since they let very few people in and assign minders to them.
It's more difficult to do that in the UK or Russia.
Of course, the intelligence services could check everyone coming in, but that's only as good as the information they have.
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