Elizabeth Drew: So how are US presidents held to account?

Donald Trump may have escaped being convicted by Congress for instigating a murderous mob attack on America’s Capitol, yet it seems clear that he will be held accountable for it in history.
The US Senate’s failure to convict Donald Trump for instigating the January 6 riot in the Capitol, for which the House of Representatives had impeached him, leaves the question of whether the US Congress has any effective means of holding a president to account for acts against the Constitution.
The nation’s Founders had sought to prevent a president from enhancing his own powers to the point of becoming, in effect, a king. Under Trump, America’s constitutional system had a dagger pointed to its heart: a president who refused to recognize that he had lost an election and was willing to use a mob to physically attack a supposedly co-equal branch.