I’m often asked why the Trump era is so messy, so acrimonious. One obvious (and well-analyzed) reason is the extreme disappointment — frustration — anger felt by the American Left, especially by those who readily identify with the resistance.

Indeed, the daily vitriol directed at the president is high-octane, relentless. But the remainder of my answer is more rhetorical in nature: “What part of ‘disrupter’ did you expect to be pleasant?”

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This response focuses on an often-neglected point: Disrupters are often uninvited, unappreciated guests. They have little respect for tradition. They revel in upsetting the apple cart, personal feelings be damned.

But it’s not just “feelings” in their crosshairs — it’s also assumptions and accepted premises, the traditional rules of the road valued by establishments everywhere.

Suffice it to say no modern politician (let alone president) has gone about the business of challenging so many assumptions and accepted premises in so brief a period of time.

For context, try on these familiar D.C. storylines:

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NATO. This bastion of the Western alliance has traditionally been immune from substantive criticism — until Trump inconveniently pointed out how a number of member states were failing to meet their promised level of defense spending. He also questioned the continued validity of (“one in, all in”) Article V, just in case his initial shot across the bow was ignored.

More recently, the president took Germany to task for its heretofore virtually unpublicized and internationally undebated building of an energy pipeline with the Russians, while falling behind with respect to its NATO commitment. The foreign policy Establishment condemned the intemperate Trump. Our embarrassed allies were none too amused.

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Tariffs. Generations of free-market Republicans have been conditioned to champion free trade — even when reciprocity is not so forthcoming or when unfair trade practices (state subsidization, material stealing of intellectual property) accompany the trading regime.

Indeed, a traditional Republican president would never countenance talk of a “trade war,” but then along comes Trump with his “America First” salesmanship and “Why not a trade war?” tweet.

To no one’s surprise, said tweet caused heartburn at the U.S. Chamber of Commerce and The Wall Street Journal, and with free-trade advocates everywhere.

It remains to be seen how long the most serious trade skirmish (with China) will last, and there is no doubt some Trump (Midwestern) constituencies have been hurt by trade reprisals.

Still, friends and foes alike should suffer no illusions about the president’s intent to challenge long-honored and well-established trade practices and procedures.

North Korea. Treading lightly with the nuclear-armed hermit state has produced nothing but decades of bipartisan foreign-policy failure. Hand-wringing on both sides of the aisle has in fact only encouraged the communist regime to engage in further provocations, including, but not limited to, a 2017 missile launch over Japan.

Still, under no circumstances would the establishments of either major political party entertain the idea of insulting the unpredictable, nuclear-armed dictator, Kim Jong-un.

But that is precisely what Trump did. And his “Little Rocket Man” and “My button is bigger than your button” rhetoric resulted in … a summit … and now the first halting steps on the long road to a denuclearized Korean peninsula.

Any progress on this genuinely worrisome front has to be the most shocking foreign policy development to date. Talk about throwing away the diplomatic rule book …

The FBI.  My earliest television memories are of Sunday nights spent with Efrem Zimbalist Jr. and the television program “The FBI.” They were the good guys. They placed themselves in harm’s way to protect us.

They were the best of the best. The G-Men engaged in the business of taking down the Mafia, drug cartels, and bank robbers. My experience as a member of Congress and as governor of Maryland only strengthened my fandom. My interactions with the agency were always professional, always focused on the job at hand. Love those guys.

Now fast-forward to the Peter Strzok and Lisa Page email saga. Include Strzok’s recent appearances before Congress. Throw in former FBI Director James Comey’s politically motivated fecklessness and that phony dossier (paid for by Hillary Clinton’s 2016 campaign and the Democratic National Committee).

The dossier was used as a predicate to conduct a counterintelligence operation into the Trump presidential campaign. The result is a big political mess and major black eye for the once revered, post-Hoover FBI.

In response, the president openly criticizes the leadership of the agency for its political manipulations during the 2016 campaign. This, of course, is unheard-of behavior …but right out of Trump’s anti-Establishment playbook. You recognize the fearless response: Hit back, hard and fast, even when the intended target can help prosecute you.

Growth. Perhaps the least provocative challenge issued by the Great Disrupter was to question the progressive Left’s retreat to 2 percent annual growth as the “new normal.”

This was the target adopted by President Barack Obama while he went about the business of apologizing to the world for America’s mass consumption economy.

Many conservative politicians and pundits took Trump and his allies to task for this limited horizons narrative.  But leave it to the consummate salesman to throw out the “four-word” — 4 percent annual growth — as reachable and sustainable. Note: Establishment economists were eerily silent as the U.S. growth rate surged to 4.1 percent in the second quarter.

Beware of traditional assumptions and premises in the Trump era.

Former Gov. Robert Ehrlich was Maryland’s chief executive from 2003 to 2007. He previously served four terms in the U.S. House of Representatives from Maryland’s second congressional district. He is the author of “Bet You Didn’t See That One Coming: Obama, Trump and the End of Washington’s Regular Order.”

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