Whoever knowingly provides material support or resources to a foreign terrorist organization, or attempts or conspires to do so, shall be fined under this title or imprisoned not more than 20 years, or both, and, if the death of any person results, shall be imprisoned for any term of years or for life. -- 18 US Code § 2339B
As Major Danny Sjursen, US Army (retired) writes:
War crimes cases aren’t supposed to be popularity contests; they are careful legal processes with specific purposes: to enforce discipline and humanity, as well as to avoid alienating the indigenous population. That’s the cardinal rule in counterinsurgency (COIN): Don’t do anything to reinforce the enemy narrative and thereby fill their ranks with new fighters. Some guerrilla war aficionados within the military have even taken to calling the concept COIN math.
Seen in this light, as a result of these pardons (and other actions), Trump acts as an unpaid “terrorist” recruiting sergeant.
Specifically, a recruiter for the Islamic State of Iraq and the Levant, which appears on the US Department of State's list of Foreign Terrorist Organizations, via his pardon of convicted US war criminal Edward Gallagher.
That's a crime under the aforementioned US Code article ("service" is one of the categories of "material support"), and since the particular form it takes is something only the president of the United States can do, it's by definition a "high" crime.