Those who follow the worldwide smartphone operating system market probably know it is essentially a two-system race. Mostly dominated by Google's (NASDAQ:GOOG) (NASDAQ:GOOGL) open-ended Android OS, with Apple's (NASDAQ:AAPL) iOS competing in select developed markets, worldwide OS market share has been rather predictable over the past year. For perspective, Android held a near-70% of the European market share, while iOS claimed roughly 18% in January 2014, according to Kantar Worldpanel. 

If there's an exception to this worldwide narrative, however, it has to be the U.S. market, where the race is much closer. Kantar estimated that Android held 50.6% of the U.S. market share in December 2013, while iOS came in at 43.9%.

However, after Apple's stellar 2014 performance in developed markets -- mostly at the expense of Android -- Cupertino can now claim the top spot in the U.S. smartphone OS market for the first time since late 2012.

One-tenth of a percent
By the slimmest of margins -- one-tenth of a percent, 47.7% versus 47.6% -- iOS edged out Android in the important U.S. market in December 2014, as sales from Apple's wildly popular iPhone 6 and iPhone 6 Plus lines led to a 3.8 percentage point increase in market share. It appears that market share came primarily at the expense of Android, which lost roughly 3 percentage points in the U.S. The chart below, courtesy of Kantar, provides more detail:

While many people believe Apple doesn't care about market share -- I disagree -- the company's investors should be encouraged that iOS is stealing market share from Android as U.S. smartphone penetration slows. For Android makers, this is a discouraging trend that should be addressed. As far as Android vendors go, there is no bigger loser in the migration to iOS than high-end device maker Samsung. Samsung famously struggled during 2014 as its Galaxy S5 sales failed to meet expectations. .

Europe isn't much better; Asia remains split
Unfortunately for Android, the big five markets of Europe -- Italy, France, Spain, Great Britain, and Germany -- didn't bring better news. Although iOS did not overcome Android in those markets as a whole, mostly because it started from a lower market share figure, Apple did increase its market share by 6.2 percentage points in 2014, from 17.9% to 24.1%. Meanwhile, Android fell 3.8 percentage points to take 66.1% of the market. In a broad-based decline, Android only increased its market share in one of the five countries -- Italy -- on a year-over-year basis.

In the key Asia markets of China and Japan, both operating systems had positive takeaways. In Android-dominated China, Google's operating system suffered only a year-over-year 1.6 percentage point decrease, to 77% market share, as Samsung and Xiaomi continue to sell well. Apple's successful launch of the iPhone 6 and iPhone 6 Plus pushed its market share in China up to 21.5% from 19%.

On the surface, Japan appears to be Apple's Achilles' heel, as it lost 8.5 percentage points of market share year over year to lead with 60.2% share. However, this was mostly due to an unfavorable comparison as the iPhone was new to major Japanese carrier NTT DoCoMo in the prior year. That said, Android picked up 7.8 percentage points during that period and commanded 38.3% of that market as of December 2014..

Where do we go from here?
iOs overtaking Android in the United States last month has a full-circle feel to it. In January 2013, Android first took the lead from iOS to top the U.S. market for that year and most of 2014. However, after Android peaked at 63.6% in August 2014, Apple has clawed its way back. While these figures might change during 2015 as new phones are introduced and upgrade cycles occur, Apple is working with a strong tailwind.