Health care was the defining political battle of President Barack Obama's first term, and — after the economy— it remains his most complicated policy challenge at home, central to his place in history.

Fixing the system is a tall order as "Obamacare's" major coverage expansion takes effect this year and next. The U.S. is projected to spend $2.9 trillion on health care in 2013, approaching one of every five dollars in the economy, much more than any other advanced country. But Americans are not appreciably healthier and more than 48 million are uninsured. The nation's mix of private insurance and government programs supports many of the world's best hospitals, but overall the quality of care is uneven and much is wasted by lack of coordination and overtreatment, putting patients at risk. Fraud bleeds the system of tens of billions of dollars a year.

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