Valve’s annual Dota 2 tournament — The International — is now one of world’s biggest sporting events. Thanks to the Valve’s most excellent shepherding of both the game and the 5-versus-5 MOBA genre, an ingenious crowdfunding scheme, and the continuing growth of spectator esports, The International now has a total prize fund of over $6 million. The winning team will take home somewhere in the region of $3 million. To put that into perspective, that’s more than all but the top sports professionals take home in a year. The Masters golf tournament, one of the world’s top sporting events, only offers around $1.6 million to the winner, from a total prize fund of $9 million. Dota 2 is now officially playing in the big leagues. How did Dota 2, which only left beta testing last year, become one of the world’s biggest sports in the world?

If you haven’t played Dota 2 before, here’s a brief description of the game. It’s free to play, for Windows, Mac, and Linux. Each game lasts for around 40 minutes, with two groups of five players working cooperatively to destroy the enemy’s base. Each player picks from around 100 heroes, each with different spells and abilities. During the game, your hero gains experience and gold, which can be spent on gaining levels (and thus abilities) and magic items (which increase your strength, your speed, etc.) It is probably the best game ever made, if you’re into cooperative gameplay, but with the caveat that it has one of the most punishing learning curves ever devised. I have played the game and its predecessor (DotA) for around eight years — and I’m still not very good.

The International is an annual tournament, now in its fourth year, where the top Dota 2 teams fight to be crowned the best in the world. In its first two years, Valve put forward $1.6 million of its own money as a prize fund, with $1 million going to the winning team. Last year, Valve introduced the compendium, an interactive digital guide for the tournament that cost $10, with $2.50 of each sale going towards the prize fund. This boosted the prize fund by around $1.2 million, to $2.8 million — again making it the largest esports tournament in the world (League of Legends had snatched it back from the first Dota 2 International with a $2 million prize fund).

How did Dota 2 become one of the world’s biggest sports?

How did Valve more than double (and perhaps triple) the size of the prize fund between TI3 and TI4? For the most part, it’s probably down to the sheer number of people playing Dota 2. This time last year, Steam reported around 325,000 concurrent Dota 2 players; today, following the end of a prolonged beta in July last year, that figure is around 800,000. In terms of monthly active users, that figure jumps to around 8 million.

Ultimately, though, the recent surge in esports popularity is entirely down to spectators. Games like Dota 2, StarCraft II, and League of Legends all have a built-in spectator mode that allow people to watch games online. This is not only a great way of learning new tricks, but also for following your favorite esports players/celebrities.

And then, of course, there’s Twitch. Twitch, which was only launched three years ago, is now one of the world’s most popular websites. The entire premise of Twitch is that millions of people stream their gaming, and tens of millions more watch those streams. Dota 2, LoL, and other multiplayer games are massively popular, but so are single-player games. In short, there are a huge number of gamers who want to watch esports — and when you have a large audience, as with NFL, baseball, or soccer, you can be sure that the big bucks will follow.

Now do you see why YouTube is acquiring Twitch?

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If you’re interested in Dota 2, The International, and esports in general, Valve’s recently released Free to Play: The Movie is a good place to start. It chronicles the first International tournament, which took place in Cologne in 2011. It’s embedded below in full.