"Whenever Trump comes up with "something", be aware, …"
A concern about the Trump statement is that either he did not understand the background of NATO, particularly article 5; a view supported by comments made by other senior US law makers - they appeared not to know either.
Or alternatively it was deliberate; a threat to NATO members and its partnership necessary for mutual defence.
The first instance would question an individuals knowledge, recall or application, critical qualities for a national leader.
The second, similar but more serious, by the apparent deliberation of the challenge - how close to the line can we go. Such a position could be against national or international law. How is an individual to be judged; someone who incites an attack on an alliance of which your country is a member, an attack on one is an attack on all, where the statement effectively encourages an attack on the homeland.
How would the US government view another national leader doing similar, inciting an attack on NATO, as an attack the US.
Whichever, the statement does not aid the collective stance required for peace management or conflict resolution between NATO and Russia.