Most of President Trump's Cabinet officials and top White House advisers fall into one of two camps. Either they're fairly conventional establishment Republicans who didn't have much connection to Trump before he was elected president (Health and Human Services Secretary Tom Price, Ambassador to the United Nations Nikki Haley) or they're people who didn't have much in the way of political experience before being appointed to serve in the Trump administration.
Some of these appointees were in business (Secretary of State Rex Tillerson). Some were in the military (Secretary of Defense James Mattis, Homeland Security Secretary John Kelly), where they had some experience with policy but much less with the political maneuvering that policy requires at the highest levels of government. Some were simply longtime Trump loyalists with little in the way of a discernible ideology, like first-son-in-law and trusted adviser Jared Kushner and Treasury Secretary Steve Mnuchin.
There are three big exceptions to this: people who were in Trump's inner circle during the campaign, but who had careers in politics and policy before Trump came along. Not coincidentally, they're also the most committed to the ideology known as Trumpism, a populist nationalism very hawkish on immigration and supportive of law enforcement as the protectors of law and order.
Two of those — Steve Bannon and Sessions protégé Stephen Miller — are in the White House. The third is Sessions, at the helm of the Department of Justice.
Sessions has decades of experience in the federal government, as a US attorney and a senator. But he'd never been in the Republican mainstream. While many in his party supported expanding legal immigration, Sessions stood firmly against immigration both legal and unauthorized; while other Republicans embraced criminal justice reforms like reducing mandatory prison sentences, Sessions remained a down-the-line "tough on crime" Republican.
Then Trump came along, and took the lead in the Republican presidential primary for taking a hard, culture-war-inflected line on immigration and a populist tone — a chance to take Sessions-ism mainstream.
Sessions was the first member of the Senate to endorse Trump, and helped the candidate shape his campaign platform; by Election Day, he was arguably the president's closest adviser. He was rewarded with the attorney generalship.
Almost immediately after Sessions was confirmed, though, he recused himself from the Russia investigation after reports surfaced that he'd met with Russian Ambassador Sergey Kislyak during the campaign, and hadn't reported those meetings during the confirmation process. Not only did Sessions not consult the president about his decision, but Trump reportedly found out about the recusal just before Sessions announced it to the public. In Trump's eyes, he's made clear, that means that Sessions's top priority wasn't necessarily protecting the president.
The extent to which Sessions has honored that recusal, especially when it comes to his involvement in firing Comey, has been debated. But the fact remains that he decided early in his tenure to sacrifice some control over a part of his job that would have allowed him to protect the president, in order to put a controversy behind him. That's the action of someone who understands the way the Washington news cycle operates, and is sensitive to losing influence with members of his party in Congress (and officials in his own department) because he's embroiled in scandal. But more fundamentally, it's the action of someone whose top priority is to enact a policy agenda, rather than simply protecting his commander in chief.