Hello there.
This is my fist answer so it will be short and simple. I went to CMU to be a history teacher and while I do not teach now, I am still a historian at heart and in my off time.
The main reason English is so widely used to day is because of the the Untied Kingdom and the British Empire.
MAP http://i.imgur.com/sCFNSrq.png
Between the 16th and 18th century, during the age of exploration, France, Holland, England, Portugal, Spain and several other nations began to 'discover' and claim land as part of their empires. At first this started with coastal communities and easily accessible regions, but as technology developed and more permanent settlements where established, more land became 'owned'. As the powers of nations such as France waned, England stood strong and continued to survive.
Eventually in the mid 19th century, English surpassed French as the main language of diplomacy and economics. The Pound Sterling became an internationally recognized currency and the world was on the cusp of the golden age of the British Empire under Queen Victoria.
Because the British Empire was the largest empire in history and lasted well into the mid 20th century, the English language dominated as universities and education in British territories was in English, news, trade and politics was all conducted in English. This structure set up a world where if you wanted to be successful or if you wanted to make your way in life under British rule, you had to know English.
What finalized the English language being the primary language of economics and diplomacy was WWII and the rise of the United States, the dollar and American foreign policy. Throughout the mid 20th century the USA was the leader in economic growth, military power and was the primary counter to the Soviet Union. Nations that wanted to play on the world stage and gain influence with Western Europe and the US had to have politicians that understood and could converse in English. With the history of the British Empire in Africa, Asia, the Middle East and the Caribbean, the foundation was there for English to continue to be the dominate language of the world.
I hope this very brief summary helps.
For additional information see below:
Across cultures, English is the word, By SETH MYDANSAPRIL 9, 2007, NY Times http://www.nytimes.com/2007/04/09/world/asia/09iht-englede.1.5198685.html
The Rise of English: The Language of Globalization in China and the European Union, Anne Johnson Macalester College 2009. Macalester International Volume 22 The Macalester/Maastricht Essays Article 12 http://digitalcommons.macalester.edu/cgi/viewcontent.cgi?article=1447&context=macintl
Edit: I updated to a historically accurate map. I also added some sources for modern day studies on the continued spread of English.
