Tonight, the chaos erupting in South Korea, a major U.S. ally, after the country's president declared martial law,

violent clashes breaking up between protesters and troops, helicopters flying over parliament.

After the president's surprise TV announcement declaring martial law, accusing his political foes of sympathizing with North Korea.

But hours later, the president backing down, our live report just ahead.

Also tonight, another massive pile-up involving cars and semis as lake-effect snow pounds the Great Lakes and northeast.

A train slamming a semi-truck in the slush and the new snow threat after several feet have already fallen.

President-elect Trump agreeing to allow the FBI to begin background checks on his staff picks.

Also, the growing skepticism among Republicans over whether Mr. Trump's pick for defense secretary will be confirmed. And our new reporting, the concerns among his former Fox News colleagues about his alleged alcohol use.

New details about that stowaway who bordered a flight from JFK to Paris.

How she was allegedly able to sneak past critical parts of security at one of the country's busiest airports.

Our underwater journey with the endangered otters guarding the so-called redwoods of the sea

and the one-of-a-kind pharmacy that dispenses prescriptions for the soul.

This is NBC Nightly News with Lester Holt.

Good evening and welcome.

U.S. officials tonight watching with great concern as a serious crisis rapidly engulfs

one of America's most loyal and strategically important allies.

Tonight, South Korea, on edge, its national assembly surrounded by protesters where there were clashes with security forces.

After the country's president declared martial law and mobilized the military, citing the threat of anti-state forces and threats from neighboring North Korea.

Opposition lawmakers moving quickly to lift martial law, but not before the government

announced a ban of all political activities and declared media and publications would be subject

to government control. The crisis catching the U.S. with 30,000 troops stationed there off guard.

NBC's Beijing-based correspondent Janice Mackie Freyer joins us now.

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