Booting Mooch was the easy part — hardly a fair fight for a four-star general.

Kelly is an almost perfect lab test of whether a Trump White House can be functional. He has all the ingredients to succeed: He's been granted unprecedented authority (for Trump), he's revered by all internally and has no dog in the factional wars in the West Wing. If he fails, we'll know this White House is truly ungovernable.

Here are five factors that'll help determine how the history books (and imagine how many there'll be!) treat White House Chief of Staff John Kelly:

Can he attract hefty new talent? (And how long can he keep the few gems that he has?) One of the corners this White House has painted itself into is the inability to get sane, successful, proficient Republicans to join the Trump show, either to fill vacant slots or as replacements in the coming wave of departures (which happens after Year 1, regardless of who is president). Can he engineer a big Hill win? Kelly is experienced in working the Senate from his days as the Marine Corps commandant's legislative assistant (2004-07), and he 's going to find extreme unhappiness with this White House's approach to policy, messaging, strategy and tactics. Can he shield POTUS from Mueller? We're going to have document and interview requests, perhaps pleas of the Fifth by people close to Trump. The special counsel has a rich new vein to exploit with today's WashPost front-pager (denied by Trump's legal team): "Flying home from Germany on July 8 aboard Air Force One, Trump personally dictated a statement in which Trump Jr. said that he and the Russian lawyer had 'primarily discussed a program about the adoption of Russian children.'" Can he mobilize this thinly staffed, inexperienced administration for a genuine crisis? The fact that there hasn't been a transcendent national security event, at home or abroad, is a miracle that can't last. With the military provocation by North Korea and the diplomatic aggression by Russia, he may get his chance quickly. Can he restrain President Trump from petty tussles, pointless tweets and random self-indulgences? Kelly has certainly marked his territory. But Trump is the ultimate boss. And as we've been telling you for a year: If there's one guy who's not changing, it's the 70-year-old billionaire with his name on the building.

Be smart: People in the room tell me that Kelly's first test will be when something bad happens and POTUS wants to react, when he'd be better off holding his fire.

Keeper quote ... An eternal Trump truth, tweeted post-Mooch by Maggie Haberman: "There are two types of non-family members in Trumpworld. Permanents & instruments. Sometimes instruments mistakenly think they're permanents."