The United States has become more dependent on local security forces in many areas. These personnel sometimes refuse to use force – or are ordered not to – against fellow countrymen. And it can be hard to stop determined protesters from scaling embassy walls, as occurred in Egypt and Yemen.

Security can easily change with a country’s politics.

In Egypt, some of the protections given to the U.S. Embassy during the tumultuous 2011 revolution that ousted U.S.-backed President Hosni Mubarak were loosened after the June election of Mohammad Morsi. A leader of the Muslim Brotherhood, Morsi has cooled ties with the United States in an apparent reflection of anti-U.S. and anti-Israeli sentiments that Mubarak stifled with force.

Some Egyptian army checkpoints on approaches to the U.S. Embassy, which sits just blocks from the revolution’s heart of Tahrir Square, were removed after Morsi assumed power. All of them appeared to have been pulled on Tuesday night, when protesters scaled the wall and placed a ladder against a flagpole, ripped down the American flag and replaced it with an Islamic banner.

Local guards also seemed to have disappeared, according to several officials inside the building.

It was only after what was reportedly a brusque telephone call from President Barack Obama that Morsi on Thursday began publicly and repeatedly condemning the violence in what seemed to be an attempt to make up for his hours of silence. By nightfall, thousands of police were posted around the U.S. compound.

Embassies must conform to special design standards that emphasize fortress-like architecture and iron gates over aesthetics. And in countries like Afghanistan, Yemen and Pakistan, they bristle with blast barriers, razor wire-topped fences, sand-bagged machinegun posts, and metal detectors that stoke anti-U.S. indignation.

While improving security for American diplomats and other staff, such precautions make it harder for diplomats to travel and meet the people with whom they are supposed to interact.

“There is an inherent conflict between assuring real security, particularly in war zones, and the ability of diplomats and civilians to do their jobs effectively,” Susan Johnson, the president of the American Foreign Service Association, told the June 2011 Senate committee hearing. “To find the right balance between the two imperatives is difficult.”

Nancy A. Youssef in Cairo and special correspondents Adam Baron in Sanaa, Yemen, and Alan Boswell in Nairobi, Kenya, contributed.