Ex-US might look unappealing now if you simply look at a back test. However, to some of the more knowledgeable, that bad looking back test can be a very good thing.

For reasons on why diversifying beyond the US can be very important, please see:

• https://www.bogleheads.org/wiki/Three-fund_portfolio

• https://www.bogleheads.org/wiki/Domestic/International

• https://www.fidelity.com/viewpoints/investing-ideas/international-investing-myths if that link doesn't work: https://web.archive.org/web/20201112032727/https://www.fidelity.com/viewpoints/investing-ideas/international-investing-myths (Archived copy from Archive.org's Wayback Machine)

• https://www.pwlcapital.com/should-you-invest-in-the-sp-500-index

• https://www.evidenceinvestor.com/which-country-will-outperform-next-is-irrelevant/

• https://movement.capital/summarizing-the-case-for-international-stocks/

• https://www.callan.com/wp-content/uploads/2018/01/Callan-PeriodicTbl_KeyInd_2018.pdf (PDF) or https://www.callan.com/wp-content/uploads/2020/01/Classic-Periodic-Table.pdf (PDF) or the archived versions if those don't work: http://web.archive.org/web/20201212205954/https://www.callan.com/wp-content/uploads/2018/01/Callan-PeriodicTbl_KeyInd_2018.pdf (PDF) & http://web.archive.org/web/20201205183933/https://www.callan.com/wp-content/uploads/2020/01/Classic-Periodic-Table.pdf (PDF) (Archived copies from Archive.org's Wayback Machine)

• Of rolling 10 year periods since 1970, EAFE (developed ex-US) has beat the S&P 500 over 45% of the time: https://www.tweedy.com/resources/library_docs/papers/Dichotomy%20Btwn%20US%20and%20Non-US%20Mar2022.pdf (PDF) or for the archived version: https://web.archive.org/web/20220501183228/https://www.tweedy.com/resources/library_docs/papers/Dichotomy%20Btwn%20US%20and%20Non-US%20Mar2022.pdf

• https://www.vanguard.com/pdf/ISGGEB.pdf (PDF) or the archived version if that doesn't work: https://web.archive.org/web/20210312165001/https://www.vanguard.com/pdf/ISGGEB.pdf (PDF)

• https://www.schwab.com/resource-center/insights/content/why-global-diversification-matters or if that link doesn't work: https://web.archive.org/web/20190124072925/https://www.schwab.com/resource-center/insights/content/why-global-diversification-matters

• https://fourpillarfreedom.com/should-you-invest-internationally

• https://mebfaber.com/2020/01/10/the-case-for-global-investing

• https://www.reddit.com/r/Bogleheads/comments/ii0sa2/considering_usonly_investing_start_here/

• https://investor.vanguard.com/mutual-funds/profile/portfolio/vtwax - Global market cap weights. I'd argue this should be the default position.

• https://investor.vanguard.com/investing/investment/international-investing - Vanguard 40% of stock is recommended to be international. This is what both Fidelity and Vanguard use in their target date funds.

• https://twitter.com/mebfaber/status/1090662885573853184?lang=en with this reply: https://twitter.com/MorningstarES/status/1091081407504498688