Allen, who posted screenshots of Trump's tweets announcing the ban on Instagram, captioned the post by saying the decision was "insane."
"Why does it matter how people identify themselves??? Anyone who wants to serve this country should have the right to serve this country," Allen continued. "What happened to the most powerful person in this country bringing everyone together?? What a shame!!"
According to CNN's Jeremy Diamond, the decision represented a reversal of a 2016 decision made by the Department of Defense under President Barack Obama that allowed transgender people to serve.
"After consultation with my Generals and military experts, please be advised that the United States Government will not accept or allow Transgender individuals to serve in any capacity in the U.S. Military," Trump said in a series of tweets Wednesday morning. "Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail."