it will involve the massive shelling of an ally's capital, which is one of the most density packed cities on earth. it would be a serious -- it would be a catastrophic war. >> reporter: kim jong-un has a million-man army and bolstered his infantry and artillery near the dmz. much of those forces are in underground bunkers, ready to fire on seoul at the first whiff of an attack by the u.s. >> the number of missiles that they have that they could launch into south korea could cause a lot of damage. >> reporter: there are about 28,000 u.s. troops in the region. the u.s. and south korea would win that war experts say, but studies suggest tens of thousands killed in the first couple of days. >> it would be a nasty tight. they have been therefore, so they know the terrain.