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Samsung has sold 5 million Galaxy S II (GS2) smartphones in 85 days, making it the fastest-selling handset in the company’s history. According to Samsung’s figures, the device sales are actually accelerating as the phone debuts on more carriers around the world. It took 55 days for Samsung to sell the first 3 million GS2s around the world, so in the past 30 days, another 2 million customers purchased the handset. With more countries waiting for the high-end smartphone, including the U.S. (which has a high smartphone adoption rate), Samsung estimates 10 million sales within seven months of the phone’s retail debut in May of this year.

What’s driving sales of the GS2? Although the Google Android (s goog) smartphone doesn’t have every possible cutting-edge feature, it comes close. The 4.3-inch screen resolution is only 800×480, for example (lower than the iPhone (s aapl) and newer Android devices), but compensates with Samsung’s Super AMOLED Plus technology for bright, vivid colors. Samsung’s new 1.2 GHz dual-core processor powers the phone, and early reviews say it’s one of the fastest smartphones available today. The 8-megapixel rear camera with auto-focus supports 1080p video recording. And some models include a near-field communication (NFC) chip, which could be used for mobile payments through Google Wallet.

On the software side, Samsung has refreshed its pleasing interface, known as TouchWiz, to hide some of the blandness of Android in its vanilla state. Samsung has also developed its own media ecosystem. The GS2, for example, can rent movies, buy music or purchase e-books directly through Samsung’s own media hubs, which are pre-installed on the smartphone. It’s worth noting that Samsung’s user interface, along with the overall look of the phone, is why Apple sued Samsung in April. That case is ongoing, and not likely to be resolved this year unless the two parties settle out of court.

Although Apple and Samsung are fighting in court, a real war looks to be shaping up between the two in the U.S. over the next few months. Apple is expected to announce and begin selling the next iPhone, which is sure to be the first with a dual-core chip, in addition to other improvements. At roughly the same time, U.S carriers will likely introduce their own versions of Samsung’s Galaxy S II, making for a head-to-head battle when it comes to smartphone sales.

For some consumers, this will be an easy choice: If they want iOS, they buy an iPhone; if Android can meet their needs, the GS2 is a top contender. But other, non-platform factors will be in the mix. AT&T (s t) is rumored to be launching a Galaxy S II with a hardware keyboard, which could appeal to some as a differentiating and desirable feature not offered by Apple. And unless Apple builds an iPhone that works on either T-Mobile’s or Sprint’s (s s) networks, those carriers’ customers have no iPhone to even consider, making the GS2 even more appealing.

While the entire smartphone market is worth watching, the war between the next iPhone and Samsung’s Galaxy S II has my full attention. Samsung has taken a very Apple-like approach with the Galaxy lineup, and it has reaped great rewards; the company was essentially neck-and-neck with Apple in terms of sales for the last quarter, with both companies dethroning Nokia (s nok). I’m sure the next iPhone will impress — the new iOS 5 software alone has already impressed me personally, and I’m an Android owner — but the Galaxy S2 is shaping up to be the key to Samsung becoming the smartphone king of the world.