Professor David Crystal has written a whole book on this, but it is possible to provide a reasonable answer without going to great lengths. First, though, it would help to define what a global language is. Why, for example, is English widely considered to be the global language when there are far more native speakers of Mandarin?
Crystal, in English as a Global Language (Cambridge, 2nd edition, 2003), defines a global language thus:
A language achieves a genuinely global status when it develops a
special role that is recognized in every country....the notion of
‘special role’ has many facets. Such a role will be most evident in
countries where large numbers of the people speak the language...
mother-tongue use by itself cannot give a language global status. To
achieve such a status, a language has to be taken up by other
countries around the world. They must decide to give it a special
place within their communities, even though they may have few (or no)
This, Crystal goes on to say, can be done in one of two ways: either the language is given official status or
It becomes the language which children are most likely to be taught
when they arrive in school, and the one most available to adults who –
for whatever reason – never learned it, or learned it badly, in their
early educational years.
The specifics of Crystal's definition may be disputed by other academics (see 'The Handbook of Language and Globalization') but few (if any) within the mainstream would argue that English isn't a global language, even if they do not agree on what exactly that means.
On 'Why English?', Crystal provides this principal reason, applicable not just English now but other languages in the past:
A language has traditionally become an international language for one
chief reason: the power of its people – especially their political and
That has, historically, been the initial stage, but it takes more:
It may take a militarily powerful nation to establish a language, but
it takes an economically powerful one to maintain and expand it. This
has always been the case, but it became a particularly critical factor
in the nineteenth and twentieth centuries, with economic developments
beginning to operate on a global scale, supported by the new
communication technologies – telegraph, telephone, radio – and
fostering the emergence of massive multinational organizations...
Any language at the centre of such an explosion of international
activity would suddenly have found itself with a global status. And
English,.... was apparently ‘in the
right place at the right time’.... By the beginning of the
nineteenth century, Britain had become the world’s leading industrial
and trading country. By the end of the century, the population of the
USA (then approaching 100 million) was larger than that of any of the
countries of western Europe, and its economy was the most productive
and the fastest growing in the world. British political imperialism
had sent English around the globe.... During the
twentieth century, this world presence was maintained and promoted
almost single-handedly through the economic supremacy of the new
American superpower. Economics replaced politics as the chief driving
force. And the language behind the US dollar was English.
Crystal develops the last part of the above later in his book:
In the late nineteenth century and the early twentieth it was the
language of the leading economic power – the USA. As a result, when
new technologies brought new linguistic opportunities, English emerged
as a firstrank language in industries which affected all aspects of
society – the press, advertising, broadcasting, motion pictures, sound
recording, transport and communications.
Both Crystal and Salikoko S. Mufwene draw parallels with the Roman empire and Latin (while acknowledging that Latin was international rather than global), though Mufwene places a particular emphasis on colonization:
Much of the impetus that today’s globalization has given to the spread
of English is also largely attributable to the earlier role that
colonization played in expanding the language geographically and
Source: 'Globalization, Global English, World English(es)', in Nikolas Coupland (ed.), 'The Handbook of Language and Globalization' (John Wiley & Sons, 2011)
What can also be taken from this is that different regions and countries have had very different experiences and histories with English. Not only did Britain not colonize the entire world, the colonization, for example, of Australia was very different to that of Ghana. Thus, while many similar factors can be ascribed to most countries or regions, the extent of their roles in the spread of English can vary greatly.
At first glance, one might consider films and music to be important, even critical, in English becoming a global language. However, while they have certainly contributed (especially in facilitating learning for those with the desire to do so), they would have been unlikely to have had much impact on their own. Witness, for example, the widespread exposure to Bollywwod films in West Africa over several decades; whilst some of the more fanatical fans have memorized the (mostly) Hindi dialogue, it has not led to them actually learning the language (personal experience of Mali and Burkina Faso since the early 1980s backs this up). Nor did my Danish grandparents ever learn any English (OK, 10 words at most), despite listening to British and American music, and seeing mostly British TV series almost daily on TV, for many decades. In short, except in rare circumstances, movies and music are at most two among many other factors in the spread of English.
Of greater importance were (and are) computers and the internet. They were at the start primarily developed and spread by Americans, and then became a means by which millions could more easily access and study a language which could help them (for example) get a better job. But, as Crystal observes, the effect the internet will have on English as a global language in the future is hard to predict:
In a few generations’ time, the Net will not be like anything we know
today. Automatic speech synthesis and recognition will be routine, and
(notwithstanding the difficulties described on p. 27) more use will be
made of automatic translation. The arrival of high-quality immediate
translation facilities will have a major impact on the use of English
(or any lingua franca) on the Net.
A number of other reasons have been put forward as to why English has become a global language, mostly summarized in this History of English article under the section Is English Appropriate for a Global Language?, but the author concludes that
the intrinsic appeal of English as a world language is probably
overblown and specious, and largely based on chauvinism or naïveté. It
is unlikely that linguistic factors are of great importance in a
language's rise to the status of world language,...
(This site has a useful sources and links page for those who wish to explore the history of English further.) Crystal himself states:
A language does not become a global language because of its intrinsic
structural properties, or because of the size of its vocabulary, or
because it has been a vehicle of a great literature in the past, or
because it was once associated with a great culture or religion. These
are all factors which can motivate someone to learn a language, of
course, but none of them alone, or in combination, can ensure a
language’s world spread. Indeed, such factors cannot even guarantee
survival as a living language – as is clear from the case of Latin,
learned today as a classical language by only a scholarly and
religious few. Correspondingly, inconvenient structural properties
(such as awkward spelling) do not stop a language achieving
international status either.