Since 1981, Bolton has been a bristly far-right Republican insider. He's served Presidents Ronald Reagan, George H.W. Bush, and George W. Bush. In 2005 Dubya nominated Bolton as his Ambassador to the United Nations; because of his contentious nature, Bolton was never confirmed.
Bolton has long been characterized as having an abrasive manner -- one foreign diplomat described him as "rude and undiplomatic." State Department colleagues accused him of "spinning" intelligence in order to support his views.
In the arena of U.S. foreign policy, Bolton is on the conservative fringe. His career is littered with inflammatory statements: "There is no United Nations" and dismissing Palestinian claims to statehood as "a ploy." He opposes the European Union; in 2008, he urged Ireland not to join the EU and, in 2016 , urged England to leave. Bolton stakes a position that's similar to Trump's "America First" stance; he is skeptical of international law and most international organizations. Bolton describes himself as a "unilateralist."
Before his appointment as National Security Adviser, Bolton split his time between legal work in Washington DC, commentary on Fox News, and consulting work for conservative organizations such as the American Enterprise Institute and the Gatestone Institute. In addition he ran the John Bolton PAC with major support from Republican oligarch Robert Mercer.
Trump's foreign policy is "personal" rather than ideological; he seems intent on undoing every accomplishment of Barack Obama. In contrast, Bolton is deeply ideological.
Iran: Trump was against the Iran agreement -- technically the "Joint Plan of Action" (JPA) signed November 24, 2013 -- because Obama was for it. Bolton has been against the JPA since it was negotiated; calling it a "massive strategic blunder." He's advocated Iranian "regime change" and is a long-time of the Iranian dissident group Mujahedeen Khalq (MEK). (https://www.nytimes.com/2018/05/07/world/middleeast/john-bolton-regime-change-iran.html )
Trump wants to negotiate a new JPA that specifically limits Iranian missile testing, gives inspectors unfettered access to Iranian military bases, and extends the (old) JPA's expiration date beyond 2030. It's unlikely that either Bolton or Trump can gain the support of the other signatories: China, England, European Union, France, Germany, Iran, and Russia.
North Korea: Trump has seized on "denuclearization" of North Korea as his signature foreign-policy initiative. Once again, his motivation is personal; he wants to succeed where Barack Obama failed. He revels in the notion that the June 12th meeting with Kim Jong Un will provide a diplomatic breakthrough and ensure his nomination for the Nobel Peace Prize.
Bolton has a long record of antagonism towards North Korea; during his tenure in the George W. Bush Administration he advocated that it be added to the "Axis of Evil." In February, before he became National Security Adviser, Bolton wrote an editorial ( https://www.wsj.com/articles/the-legal-case-for-striking-north-korea-first-1519862374) building the case for a preemptive strike against North Korea.
In early May, it was reported that Trump had discussed removing all US troops from the Korean peninsula; it's likely that ultra-conservatives, like John Bolton, talked him out of this. On May 13th Bolton offered a very specific definition of what North Korean "denuclearization" meant: “getting rid of all the nuclear weapons... taking them to Oakridge, Tennessee... getting rid of the uranium enrichment and plutonium reprocessing capabilities.” Bolton said the process should follow "the Libya model."
North Korea immediately rejected Bolton's comments: "This is not an expression of intention to address the issue through dialogue. It is essentially a manifestation of awfully sinister move to impose on our dignified state the destiny of Libya or Iraq which had been collapsed due to yielding the whole of their countries to big powers.” At this writing, North Korea is threatening to cancel the June 12th meeting.
Russia: To say the least, Trump's attitude towards Russia has been inconsistent. On the one hand he called Putin to congratulate him after he was "reelected" Russian president. On the other hand, he's claimed, "nobody has been tougher on Russia than I have."
John Bolton is a Russia hawk. He's accused Putin of lying to Trump about interference in the 2016 election -- Bolton is convinced Russia did interfere -- and writes, “The notion that the Russians can help us with terrorism ... is delusional."
China: To a lesser extent, Trump's attitude toward China has also been inconsistent. Bolton is a China hawk. His associates say, "The new US national security adviser is willing to risk a military conflict with China to achieve President Donald Trump’s goals for America."
Summary: On many issues, John Bolton bolsters Trump's positions with a strident unilateralism. However, Bolton appears to be mired in an old-school view of his office. He's eliminated the White House position of Cybersecurity chief. He's also disbanded the "global health security" team -- the group charged with reacting to pandemics. Bolton is far more dogmatic than Trump.
Bolton and Trump do not appear to be on the same page regarding negotiations with North Korea. It doesn't appear that Bolton will have a long tenure as Trump's National Security Advisor.