Here's the detailed argument for why I hold international stocks (VXUS) in addition to US stocks (VTI).

The US represents approximately 60% of global market capitalization. Holding only VTI means 100% of my equity exposure is in one country's companies, subject to one country's economic cycles, regulatory environment, and currency.

VXUS covers the other 40% of global market cap — developed markets in Europe, Japan, and Australia, plus emerging markets. The correlation between US and international returns is imperfect, providing real diversification benefit over full market cycles.

The pragmatic argument against VXUS: international stocks have underperformed US stocks for the last 15 years. This is true and frequently cited. It's also a 15-year sample from a period of unusual US dominance. Regressing to the mean is not guaranteed but the historical long-term correlation between US and international returns is tighter than the last 15 years suggest.

I hold 80% VTI / 20% VXUS. I don't know if VXUS will outperform going forward. I know that holding only US stocks is a large geographic concentration bet that the next 30 years will mirror the last 15.