While government employees engaged in essential national security and intelligence-gathering activities would report to work as usual - at least in the short term - many could face considerable personal hardship because of delayed paychecks. Active-duty servicemembers might be compensated; civilians, not so much.
A government shutdown would also affect U.S. foreign policy more subtly by delaying critical foreign-policy related hearings in Congress, paring back nuclear and other critical energy programs to the bare minimum, and interfering with the State Department's ability to police itself.
"Spies will still spy. The machinery will go on," said a retired senior CIA official who spoke on the condition of anonymity. "The problem is if something extra falls into the system. If guys are worried about their paychecks, they're not concentrating on their job."
The Department of Defense will likewise "continue to support all key military operations such as the war in Afghanistan and various other missions around the world," Pentagon spokesman Cdr. Bill Urban told Foreign Policy. But a shutdown would "place significant hardships on a workforce already strained by recent administrative furloughs.
For the hundreds of thousands of non-essential civilian personnel employed in the U.S. foreign policy machine, it will most likely mean being furloughed. This includes roughly 400,000 employees at the Defense Department alone.
The length of the shutdown will also partially determine its impact on the conduct of foreign policy. If it's short - two weeks or less - intelligence operations won't be slowed down or hindered. "We do a brilliant job at triage in the short term," said Charlie Allen, a retired intelligence officer whose decades-long career included assignments at the CIA and the Homeland Security Department. He noted that during the last shutdown, intelligence employees who were deemed essential, including himself, kept working.
But if the shutdown drags on, the spy agencies will feel the pinch. "If it lasts more than a couple weeks it could have extremely serious and detrimental effects upon our contractor community, and we depend on ⅛them⅜ in so many areas of our work," said Allen, now a principal at the Chertoff Group, a global security and risk management advisory firm. A wide range of intelligence operations - from launching satellites to analyzing imagery to keeping technology systems running - are outsourced, meaning that they cannot continue indefinitely without federal funding.
If contractors aren't paid or if new contracts can't be awarded, intelligence operations could slow down or be put on hold. Bloomberg Government estimates that contractors will lose roughly $1 billion per day if the government closes its doors.
Over at the State Department, operations will likely continue like normal with one major exception: The department's internal watchdog will be forced to close up shop until the money starts flowing again.
Politicians from both parties have suggested that a shutdown would prevent the State Department from handling passport and visa applications, but State Department spokesman John Gerlach said consular services wouldn't be impacted because they're funded through processing fees, not through congressional appropriations.
The impact of a shutdown would also be blunted by the fact that State and USAID have access to some residual money that has already been appropriated and given to the department. Gerlach said that some of those funds were tied to multi-year projects, while others were given to the department to spend over whatever time period it deemed necessary. Both pools of money will be available to the department after Sept. 30, 2013.
That said, Gerlach cautioned that some parts of the department would still need to temporarily halt their operations, most notably the Office of the Inspector General. That would halt any ongoing probes into possible misconduct and prevent the watchdog from launching new ones, making it much harder for the department to police itself in the days, weeks or even months ahead.
The State Department has also imposed a full hiring freeze, and would-be staffers who have received job offers but not yet started their work will be unable to do so until the shutdown ends.
Outside the handful of executive agencies that conduct the bulk of U.S. foreign policy, however, the impact of a shutdown will be more noticeable. On the Hill, many committee activities, including hearings and bill markups, will come to a halt. That includes things like the House Foreign Affairs Committee's hearing on the Somalia-based militant group al Shabaab, scheduled for Thursday, and its mark-up of a bill to reform the State Department's internal review process.
Still, the House Permanent Select Committee on Intelligence (HPSCI) will remain open for business. "The HPSCI performs crucial oversight of our nation's intelligence agencies, and those oversight activities will continue, shutdown or no shutdown," committee spokeswoman Susan Phalen told Foreign Policy. Phalen said any potential HPSI hearings or markups would not be cancelled.
The Department of Energy, which oversees the country's stockpile of nuclear weapons as well as its Strategic Petroleum Reserve, would see a particularly dramatic personnel reduction. Out of the department's nearly 14,000 employees, only a couple hundred will report to work if Congress fails to reach a deal to keep the government open. As a result, numerous programs with foreign policy relevance - such as those related to nuclear and renewable energy - will continue with skeletal employee crews. The Department of Energy's Office of Policy and International Affairs would be completely shuttered.