THREE YEARS have passed since the first sparks that led to the Arab Spring, a wave of revolt against authoritarian regimes that unleashed hopes for democracy in a region long without it. But events this week showed that Egypt has abandoned the path to democracy. This is a tragedy with serious implications for the United States.

The most damaging setback came Wednesday, when Egypt’s interim cabinet announced that it was declaring the Muslim Brotherhood a terrorist organization. This move followed a bombing Tuesday in the Nile Delta city of Mansoura in which at least 15 people died and more than 100 were injured. Without any evidence, authorities blamed the blast on the Muslim Brotherhood, which had condemned the bombing. A jihadist group, Ansar Beit al-Maqdis, claimed responsibility for it.

The violence is abhorrent, but it is clear that Egypt’s military-backed government is using it as a cudgel against the Muslim Brotherhood, the Islamist group that existed in the shadows for decades but broke into open politics with the rise of its former leader Mohamed Morsi as Egypt’s first democratically elected president in June 2012. After a miserable performance in office, Mr. Morsi was overthrown in a coup in July. Since then, scores of Muslim Brotherhood members have been killed in protests and most of the group’s leaders have been arrested. Its designation as a terror group will have a wide impact, shuttering hundreds of charities and nongovernmental organizations that are affiliated with it.

This is a huge step backward that will further alienate a broad social movement. The cabinet announcement said that authorities would punish anyone who joined or remained a member of the Muslim Brotherhood, as well as those who participate in its activities or “promotes it by speech, writing or any other means and all who fund its activities .” On Thursday, at least 16 members were arrested and the group’s newspaper was blocked from publication.

Separately, three leading secular activists who spearheaded the 2011 uprising against President Hosni Mubarak were sentenced this week to three years in prison for organizing an unauthorized march.

The Obama administration has put a brake on some military aid and loans to Egypt but has been loath to go further. In response to Wednesday’s announcement, the State Department expressed concern “about the current atmosphere and its potential effects on a democratic transition in Egypt.” Spokeswoman Jen Psaki said, “It is essential for Egypt to have an inclusive political process” and “there needs to be dialogue and political participation across the political spectrum.” All true, but these words seem awfully meek in light of what’s happened. How can there be dialogue and inclusion if the opposition is criminalized?

Egypt has long been a close ally of the United States and a keystone of stability in the region. But the time has come for stronger U.S. protests and action. To remain timid in the face of repression will invite only more.