What factors contributed to make English an international language?

The major factors that have caused English to become an
international language are political and economic.  English has become an international
language because Great Britain, and later the United States, have been the most
important powers in the world.  Their power has caused their language to be more useful
to people than any other language.

Both Great Britain and
America have created empires, either formal or informal.  The British domination of
India, for example, made English a very important language on the Indian subcontinent. 
Later, the American domination of world trade and politics after WWII made it important
that people in all corners of the world should be able to speak
English.

In these ways, economics and politics have
combined to make English the most international of languages.