When crises arise, then, there is both the opportunity and, lets face it, a clear desire for the head of state to "take charge and lead the people". If the problems of politics get in the way, then the president has a nearly irresistable opportunity to sweep those "problems" away, which usually means "emergency powers" of some sort. Once gained, these powers are very rarely given up, as there are always new "crises" to exploit to retain them.

These sorts of events are incredibly common. In fact, they're so common that the fact that the United States has never had this happen has baffled political scientists since the phenomenon was noticed. There are a number of theories as to why, but my own favorite stems from an American military tradition, which is that soldiers swear loyalty to the Constitution, not the president, despite being their "commander in chief". One of the most treasured aspects of the American system is its balance of power between judiciary, executive and legislature; while its effectiveness can sometimes be questioned, it's important in that it enshrines the idea that the United States is a country where the laws stand above the president; that the symbolic power of a head of state will never confer absolute power upon him. "L'etat c'est moi" does not apply. It is, perhaps, the only way in which one can have a powerful president without having the system fly apart in the face of crisis.