Tom Bowman

On this morning 150 years ago, Union and Confederate troops clashed at the crossroads town of Sharpsburg, Md. The Battle of Antietam remains the bloodiest single day in American history.

The battle left 23,000 men killed or wounded in the fields, woods and dirt roads, and it changed the course of the Civil War.

It is called simply the Cornfield, and it was here, in the first light of dawn that Union troops — more than 1,000 — crept toward the Confederate lines. The stalks were at head level and shielded their movements.

Gunmen wearing Afghan police and army uniforms have killed 40 U.S. and NATO troops so far this year, and the top American commander in Afghanistan says there is no single reason — and no simple solution.

Taliban infiltrators, disputes between NATO and Afghan security forces, and even the timing of Ramadan, the Muslim holy month, are all factors, according to Gen. John Allen.

"We think the reasons for these attacks are complex," says Allen, who spoke by video link from Kabul on Thursday. Ten of the American deaths have come in just the past two weeks.

The top officer in the U.S. Air Force, Gen. Norton Schwartz, is stepping down Friday after four years on the job.

Schwartz got the job after his predecessor was fired for — among other things — clashing with his Pentagon bosses over how many fighter jets the military needs.

Schwartz is most likely to be remembered for pushing another kind of aircraft: drones.

At this moment, dozens of these unmanned aircraft are flying high above Afghanistan.

Just don't call them drones when speaking with Schwartz.

"Deathly afraid."

That's what one U.S. official says about the prospect that Syria's vast stockpile of chemical weapons might be used against rebel forces. From a U.S. national security standpoint, an even worse outcome would be for those weapons to fall into the hands of terrorists.

(NPR Pentagon correspondent Tom Bowman has been embedded with U.S. troops in Afghanistan this month. On Morning Edition, he reported from the eastern province of Ghazni about what's being called "the last major combat offensive of the Afghan War." Now, he tells us about his interview with the No. 2 U.S. officer in Afghanistan, Lt. Gen. Curtis Scaparrotti.)

Inside a plywood shack at a combat outpost in Marjah, in Afghanistan's Helmand province, three Marines sit before a bank of computers provided by the military to help keep up morale. The dingy outpost is made up of a collection of tents where troops live among swarms of flies and the constant hum of generators.

Sgt. J.P. Huling, a Marine from Ohio, was killed this month in southern Afghanistan.

It wasn't a roadside bomb or a Taliban sniper that killed him. It was another sergeant — an Afghan soldier known as Sgt. Zabitollah, who like many Afghans went by one name.

It was a grim coincidence that brought these two sergeants together on May 6, a Sunday afternoon, at a mud-walled compound along a desolate stretch of road in a remote corner of Afghanistan.

When President Obama on Tuesday signed a 10-year security agreement with Afghan President Karzai, it wasn't announced how many U.S. troops would remain in Afghanistan past 2014 — the year Afghans are supposed to take over full responsibilty for security there.

There's some soul-searching going on in the military these days.

The latest scandal to hit U.S. troops fighting in Afghanistan surfaced last week when The Los Angeles Times published photographs showing smiling American soldiers holding up body parts of a Taliban suicide bomber.

U.S. Defense Secretary Leon Panetta addressed the latest incident during a trip to Brussels.

"That behavior that was depicted in those photos absolutely violates both our regulations and, more importantly, our core values," he said last week after a NATO meeting.