In theory (and face value), Nikon does offer warranties that are valid worldwide for purchases made within the market the goods were intended for, including for bodies. There's a ton of misinformation regarding so-called GREY products though and unfortunately a number of Nikon service centers that are taking advantage of consumers. Specifically, classifying certain purchases as grey when they're in fact not.

Examples:

I'm in Canada and I buy a Canadian Nikon from a Canadian dealer. The warranty applies worldwide and I can get service anywhere while, for example, traveling. Or if I move to another country, I should have no problem with service.

I buy a US Nikon from a US dealer while traveling. Same, the warranty should be fine and honored worldwide.

Now, if I were to buy a Hong Kong or US Nikon from a Canadian dealer, or a Hong Kong or Canadian Nikon from a US dealer, that's a grey market camera, a product sold outside its domestic market, whether or not from an otherwise authorized dealer. Authorized dealers are authorized to sell domestic Nikon products only. The warranty on that body is only going to be good at the place it was purchased from (if they're reputable) or the country it was originally intended (and marked) for.

That said, you will read stories online of people having issues getting repairs on their gear - this includes lenses. Nikon Canada has (or had) a notice on their web site basically dishonoring worldwide service. It's a huge slap in the face because they're marketing their own stock as having worldwide warranty coverage.