Since the onset of the Covid-19 crisis in the United States, government action taken to “flatten” the curve of disease transmission has varied dramatically among states, counties, and cities. The early epicenters — New York City, Washington State, and the San Francisco Bay Area — implemented aggressive measures in mid-March, many of which remain in place. Other states and localities opted for milder restrictions, acted much later, or barely intervened at all. Many states began unwinding restrictions weeks ago, although surging case numbers are prompting some to change course. The patchwork nature of the response helps explain the current situation: Covid-19’s spread now has many different trajectories, which partly track jurisdictional boundaries. In the third week of July, for example, Covid-19 incidence was 10 times as high in some states as in others.