Peter Bergen is CNN's national security analyst, a vice president at New America and a professor of practice at Arizona State University. He is completing a book about President Trump's national security team and policies. The opinions expressed in this commentary are his own. View more opinion articles at CNN.

(CNN)What a difference two years make. Back then, John Kelly, a former four-star Marine general, was Trump's chief of staff; James Mattis, another retired four-star Marine general, was US secretary of defense; H.R. McMaster, a three-star general and war hero with a PhD, was national security adviser; Rex Tillerson, who had run ExxonMobil, one of the world's largest corporations, was secretary of state, and Gary Cohn, the former COO and president of Goldman Sachs, was Trump's chief economic adviser.

On a routine basis, this group would debate and even challenge President Trump.

They have all long since moved on. Trump has now surrounded himself with yes-men and is running his Cabinet like he used to run his real estate company -- as a one-man show surrounded by flunkies.

Trump's key foreign policy adviser is his son-in-law, Jared Kushner.

His key economic advisers, Larry Kudlow and Peter Navarro -- who lack the stature and experience of someone like Gary Cohn -- are egging on the President to ramp up his trade war with China.

His new secretary of defense, Mark Esper, is competent, but hardly has the stature of Mattis.

Acting White House Chief of Staff Mick Mulvaney is a Trump factotum and certainly no John Kelly.

Secretary of State Mike Pompeo is "a heat-seeking missile for Trump's a--" a former US ambassador said, describing Pompeo's obsequious behavior in the recent New Yorker profile of the secretary of state.

CIA Director Gina Haspel is a CIA lifer who just keeps her head down.

Trump largely ignored his third national security adviser, John Bolton, a veteran of Republican administrations going back to Reagan, and eventually forced him out last week, in part because Bolton criticized the now-scrapped plan to host Taliban leaders at Camp David earlier this month.

Now comes Robert O'Brien, Trump's pick to replace Bolton as his fourth national security adviser. Readers can be forgiven for asking: Who is he?

O'Brien is a conservative lawyer who was appointed last year to be Trump's lead hostage negotiator at the State Department. O'Brien is favored by Pompeo, now the key player in Trump's Cabinet.

Hostage negotiation is an area in which the Trump administration can certainly claim some real success, as the President did on April 26, 2019, when he tweeted "'President Donald J. Trump is the greatest hostage negotiator that I know of in the history of the United States. 20 hostages, many in impossible circumstances, have been released in last two years. No money was paid.' Cheif [sic] Hostage Negotiator, USA!"

Speaking at a news conference in March, O'Brien said: "The President has had unparalleled success in bringing Americans home without paying concessions, without prisoner exchanges, but through force of will and the goodwill that he's generated around the world."

During O'Brien's tenure as lead hostage negotiator, North Carolina pastor Andrew Brunson was released from prison in Turkey, where he had been held on trumped-up terrorism charges. Danny Burch, an American oil worker kidnapped in Yemen, was released in February.

But it was astonishing to see O'Brien show up last month at the trial in Sweden of American rapper A$AP Rocky, who was charged with assault and whose cause had been touted to Trump by Kim Kardashian West.

This was an egregious misuse of O'Brien's time and of taxpayer dollars: A$AP Rocky wasn't kidnapped by a terrorist group or held by a despotic regime unjustly. He faced criminal charges in a closely allied, rule-of-law democracy, for which he was eventually convicted.

Meanwhile, other genuine American hostages are still in great jeopardy, such as Kevin King, a professor who was kidnapped in Afghanistan in 2016 and is seriously ill as he continues to languish in Taliban custody.

Attending A$AP Rocky's trial in Sweden is emblematic of how O'Brien will likely spend his time at the White House: as a Trump gofer.

As Trump observed earlier this month when he mused about his pick for his next national security adviser: "It's great because it's a lot of fun to work with Donald Trump. It's very easy actually to work with me. You know why it's easy? Because I make all the decisions. They don't have to work."

O'Brien is about to find out how true that is.