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More than a norm, the idea that a president-elect should defer to the sitting president in the realm of foreign policy has roots in law. A Supreme Court ruling in 1936 held that the president possesses “exclusive power … as the sole organ of the federal government in the field of international relations.”
Barbara Perry, a professor of presidential studies at the University of Virginia’s Miller Center for Public Affairs, said the ruling confirms the court’s view that “the Founding Fathers and the Constitution want the United States to speak with one voice, and that voice should be the president of the United States.”
One danger of a president-elect’s injecting himself into an active debate before he takes office is that something could go wrong(kek), raising the question of who would be accountable, Perry said. (Well, it won't, but you can worry about it. What about FDR's wife as acting president and signing laws and orders when he was very sick, wasn't that much worse?)
If Trump’s warning prompted Hamas to execute the hostages before he took office, for example, “who would be responsible for that?” she asked. (They are missing the point, they won't, because they know Trump, will fulfill his threat)
Still, presidents-elect have been hard-pressed to absent themselves from issues they’ll be inheriting before long. Dwight D. Eisenhower traveled to Korea ahead of his inauguration in 1953 to see for himself whether the war there was winnable. On the ground, he met with South Korean President Syngman Rhee, dismissed scenarios for escalating the war and came away convinced the fighting needed to end. (So they are admitting prior president elects have done it, but Trump shouldn't, hypocrites, one and all
Barack Obama was selective in how he chose to engage before he took office in the thick of a global financial crisis in 2008. When President George W. Bush invited Obama to a meeting of world leaders devoted to the downturn, Obama declined. An Obama spokeswoman said, “We really felt strong that there was only one president at a time and George Bush was the president.”
Yet Obama injected himself into legislative debates at the time, calling for extending unemployment benefits. He was more reticent when it came to fighting between Palestinians and Israelis — an intractable dispute that, of course, continues to this day.
A Washington Post article three weeks before Obama took office quoted a government professor as saying, “It seems clear he’s just cherry-picking those things that can serve his purpose and staying as far away from Middle East troubles as he can.”
https://www.nbcnews.com/politics/donald-trump/trump-isnt-president-yet-hasnt-stopped-starting-act-rcna182657