Problematic enough that Twitter had to issue a statement on why they didn't take action on it: https://twitter.com/Policy/status/912438046515220480
If you're confused or in doubt as to what Trump means in that tweet, I'm really not sure what to tell you. It's a very clear threat.
Edit: Hacker News is limiting me, so in reply to jtsummers below:
1) that’s an idealistic and unrealistic view of how international affairs work. And in fact, being able to apply such leverage actually makes war less likely since it allows countries to better gauge each other’s true intentions and willingness to act, preventing catastrophic miscalculations.
2) Congress hasn’t exercised its power to declare war since WW2, yet the US has fought in many wars since then.
3) Congress sometimes but not always has given authorization for these conflicts in lieu of a declaration. But sometimes the President has acted totally unilaterally, like in the Korean War in the 50s. The War Powers Act explicitly gives the President some unilateral authority, but...
4) The predominant legal view in the US is that the President does not need Congress’s authorization, at least to start a war, as it is inherent in his Constitutional powers.
Edit Edit: I would reply directly, but Hacker News is rate limiting me.
Also, I don’t think he is bluffing. I think Trump is dead serious about annihilating NK if they don’t disarm. If NK, either by its own calculus, or under China’s pressure, does not change course, they are toast.
I’m not saying point 4 above is correct on principle, but that it’s the view, based on the “commander in chief” clause, that will likely prevail in the chambers of the Supreme Court. That’s how law works. A lot of things would be very different if we actually adhered to the Constitution. And they’re unlikely to even take such a case for various reasons (such as the “political question” doctrine, as well as mootness).
I would really like to see your explanation for (4) given the constitution explicitly only gives the authority for a declaration of war to Congress (Article I, Section 8 in case you need the reference).
Regarding (1), if the president continues to bluff, then no nation can accurately gauge his or the US's intention. Continuing to threaten, but not seek out approval for, war only encourages NK (and others) behavior because they are increasingly confident he's all bark and no bite.