"Put it all in dog money!"
I have about half of my HSA in bonds, and a decent chunk of that is international bonds. They've done surprisingly well. For my retirement accounts, I've wondered about diversifying some of my VTSAX across a broader range, but, as mentioned above, there is an old cliche in the FIRE community that many of the companies inside of it are already international companies. Some day I would, as others have mentioned, put some allocation into an international fund, but in general, I have more faith in the US economy than most. The problem of investing in, say, some sort of worldwide index is that so many countries are relatively unstable, which makes for a good reminder that stock markets only "work" under specific conditions, so I try to make sure my life is designed to be a little more resilient, market gains or not. The other problem is that countries are so interconnected - the Great Recession in the US echoed across the world. But I still plan to diversify more some day.