On Sunday, American and British troops completed a secret withdrawal from Helmand Province, handing over one of the largest bases in Afghanistan to local troops. The maneuver effectively endedBritain's involvement in the 13-year-old war and was a major step in the drawdown of American combat troops, which are slated to leave Afghanistan by the end of the year.

While there is a surfeit of fondness for declaring a theater of battle to be "key" or "crucial" or "vital," these descriptors actually fit Helmand, especially within the context of America's longest war. Helmand, in addition to being Afghanistan's largest province, was the venue of some of the war's bloodiest action.

The most direct way of understanding the importance of Helmand may not be in the numbers of battles (many) or accompanying casualties (also many), but in what British and American forces are leaving behind. The compound hosting the adjacent American and British bases⎯Camp Leatherneck and Camp Bastion, respectively⎯is spread across about 6,500 acres of desert (or one-eighth the size of Baltimore) and housed as many as 40,000 personnel at its peak (or more than one-fourth of the total coalition forces).

As Reuters notes, the U.S. military is leaving behind "$230 million worth of property and equipment" for use by the Afghan troops, including an airstrip and a number of buildings. As The Wall Street Journal reports, the Afghan Army's 215th Corps will also be inheriting chairs, coffee machines, and a "row of cars ... with their keys in the engine, ready to be driven by Afghan soldiers."

(Related: Will Afghanistan Become the ‘Forgotten War’ Again?)

Beyond the gates of the compound sit the fields where "80-90 percent of the opium that helps finance the Taliban's insurgency" is grown. According to a United Nations report, last year that area grew to reach "a historic high in 2013 of 516,000 acres, a 36 percent increase from 2012."

In other words, the effort to cut off a main source of Taliban money goes down as an objective left unfinished by the American and British forces. The same goes for defeating the Taliban itself; many fear the group will continue to assert itself against an unsteady Afghan army. The New York Times observes that the departure of coalition troops comes during "the deadliest period on record for Afghan forces."

In the six months since March, more soldiers and police officers have died than any period since the start of the war, evidence of the grinding battle that lies ahead.

This year is also expected to be the deadliest on record for Afghan civilians.

The manner of the American and British egress from Helmand seems symbolic of both the campaign's outcome and the remaining danger. As Colum Lynch writes, despite the importance of the province, "there were no White House statements issued Sunday to commemorate the occasion, no press conferences convened to celebrate the day. Instead, U.S. Marines and British forces in southern Afghanistan quietly lowered and folded their flags in a solemn ceremony..."

By the year's end, there will be roughly 12,500 troops left in Afghanistan, charged with training and advising the Afghan army.