For an extended, excellent of analysis of 32 U.S. Code, Section 502, see https://www.brennancenter.org/our-work/analysis-opinion/presidents-power-call-out-national-guard-not-blank-check.
Bottom line, I, along with many other lawyers and legal scholars, know that Trump's use of his ordered "specialized units" is unlawful. But that makes no difference, because by the time any challenges to their deployments happens, the military will already be in U.S. streets. ... 5/