The scores of crisp American and District of Columbia flags sailing above Pennsylvania Avenue are a traditional sign of a coming presidential inauguration. This year, there are some new indicators: Thousands of armed troops and police officers, blocked-off streets and miles of barricade fencing. The bustle of the nation’s capital before an inauguration has been replaced with the eerie quiet of a city in lockdown. The deadly Jan. 6 attack on the U.S. Capitol by supporters of President Trump has turned pre-inaugural Washington into a secured encampment, one that has drawn comparisons with Baghdad’s Green Zone where foreign embassies are...