This tweet by President Trump is out of character in response to a North Korean missile launch. I believe it signals that POTUS has little confidence in existing ABM systems to handle a North Korean ICBM launched at the continental U.S.

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I think it's more that Trump will grab any stick he can to whack at someone who threatens his agenda. Even if grabbing that stick gives the impression he has little confidence in our current AMB systems and/or is more concerned about stopping Mexicans from crossing our borders than preventing ICBMs from doing so.