There is no reference to a national security document that supports Trump's claim that Greenland is necessary for US national security. An invasion of Greenland would be an act of war and under the US Constitution would require a declaration of war by the US Congress.

In the 21st, Century US presidents have declared countries to be threats without explaining why they are threats. For example, in 2015 President Obama declared Venezuela to be "an unusual and extraordinary threat to US national security" without providing any explanation of the threat.

Some Russians, who still hope for a Washington-Moscow peace agreement, interpret Trump's threat to Greenland as Trump's intent to break up NATO and thereby arrive at a peaceful solution with Russia.

European NATO members, according to news reports, are discussing the possibility of sending military forces to Greenland's defense.

Most likely this is Trump issuing a threat that he will withdraw in exchange for an agreement about Greenland. However, Trump has said that he "doesn't need" international law and is bound only by his "own morality." "My own mind is the only thing that can stop me."  in other words, Trump is a law unto himself.

Trump has also ordered Cuba to make a deal "before it is too late." Militarism is in the picture, but it is unknown whether it is real or an intimidation tactic.

Reports conflict on whether intimidation is succeeding with the Venezuelan government.

Trump's approach is not diplomatic and displays his firm belief in US hegemony. Will Russia, China, and Iran notice, or are their heads in the sand like ostriches?

Can Trump enforce his threats over a wide area of the earth from Latin America to a NATO member to Iran? If not, will the US be isolated by Trump's threats?

Trump's latest excuse for taking Greenland is to prevent Russia and China from doing so. The head of Denmark's defense committee said that it is delusional to think that China and Russia have a military presence in Greenland and intention to seize the Danish territory.