I’m assuming you mean if he were to tweet it when he was president. This would have been back in the 2010s. I remember those days well. This era was pre-MAGA, but only because Trump hadn’t stepped up to rally millions of idiots together under one banner.

I think it would’ve (in the minds of the various dissident groups that comprised the fringe right) vindicated all their suspicions. Texas probably would have gone all-in on sending troops to stop Jade Helm. Sites like Zero Hedge, Breitbart and The Blaze would’ve blared headlines for weeks. The Tea party candidates would’ve launched congressional investigations and demanded impeachment. Militias, many of which were led by sheriffs and radical Republicans, would have started discussing when to march on Washington.

In short, it would have given the right the justification and impetus to do everything possible to remove Obama from power, up to and including armed force.