A soldier posts a video criticizing the president. A Marine tweets her opinion on military policy. A Navy captain—now a United States senator—reminds active-duty personnel they can refuse unlawful orders. Each of these acts raises the same fundamental question: where exactly does the First Amendment end inside the military?

The answer is more complicated than most service members realize, more consequential than most civilians understand, and more unsettled than any branch would prefer to admit. The stakes—careers, pensions, liberty—demand that every service member know the terrain before they speak.