A more thorough explanation:

Kleindienst v. Mandel is a legal case that was decided by the United States Supreme Court in 1972. The case involved an alien, Ernest Mandel, who was invited to speak at an academic conference in the United States. However, he was denied entry into the country because of a law that excluded aliens who promoted communism.

The Supreme Court held that the Attorney General had properly exercised his discretion in denying a waiver to the exclusion. The Court declined to examine the substance of the Attorney General’s decision or to engage in balancing the decision against the First Amendment rights of the U.S. citizens who had invited the alien to speak at the conferences.

For example, Ernest Mandel, a Marxist scholar from Belgium, was invited by Stanford University to speak at an academic conference. However, he was denied entry into the United States because of the law that excluded aliens who promoted communism. The Supreme Court held that the Attorney General had properly declined to waive the statutory exclusion and that the U.S. citizen plaintiffs’ First Amendment rights had not been infringed upon.

The case reaffirmed that the power to exclude aliens is a power inherent to every sovereign state. Congress has the right to exclude aliens and to delegate discretion to the Attorney General to waive the exclusion. The Court held that because the Attorney General’s decision not to waive was “facially legitimate” and “bona fide,” the Court would not consider the substance of the decision, nor would it engage in balancing against the First Amendment interests of the U.S. citizen plaintiff-appellants.