Since World War II the economies of individual countries have merged into continental economies, and from there have grown into a global economy. The connections between the world's markets were clear for everyone to see. Since then, the ties between the world's economies have grown stronger. Part of the reason for this is the advance of electronic communications. An event 100 years ago that might have taken days to affect the world's economies now rushes around the world with lightning speed.
Computers track markets constantly and detect the slightest fluctuations.
What happens on Wall Street can affect markets in Tokyo, Bonn, and Sydney almost instantly. What happens in Singapore, Hong Kong, or London can push up or pull down the value of investments around the world. Investments everywhere are interconnected in a complex web of buying and selling that doesn't recognize the boundaries of countries. To limit yourself to merely investing in one country means greatly limiting your investment opportunities. International diversification provides a variety of potentially high-growth investment choices in stocks and bonds.