Ooh, I get to choose the location?
Fairly close to the North Pole. Our people are adapted to it, assuming the island has always been there somehow. Nobody else is, making the logistics of trying to invade an absolute nightmare for anyone on Earth.
Now we just have to feed ourselves somehow in the event of a naval blockade, so a lot of investment into ballistic missiles and submarines to make a blockade as difficult as possible and hopefully keep our trade open during the summertime. I assume we're fully reliant on imports that can only come in during that seasonal window. Fishing alone probably can't sustain a population that large, and nothing is going to grow up there.
Assuming an invasion has to occur in 10 years and cannot be delayed, whoever tries is kinda fucked though.
If North Pole is cheating, then in the middle of the Pacific with the same plan. Probably can't resist something like the US though in that case.
Sea levels rise the same everywhere. And not in 10 yrs it won't.