The U.S. is a key international player, but it's not alone, as one global crisis after another in the past 15 years has moved markets around the world.
Dale of Security Bellew says one third of long-term investors' equity allocation should in both developed and developing markets.
Note that during much of the last decade international stocks outperformed U.S. ones.
During the commodities boom, countries rich in natural resources — Australia, Canada, Brazil, South Africa — had booming economies, which boosted the value of their stock markets and currencies.
More recently, investors who wanted sovereign debt other than U.S. Treasurys shied away from EU countries and gravitated to emerging markets.
"There are opportunities in some emerging debt," notes Phil Cook, founder of Cook & Associates, a financial planning firm in Manhattan Beach, Calif.