Brandon Daugherty of the Air Force’s 308th Rescue Squadron helps evacuees board a helicopter on Wednesday in Port Arthur, Tex. (Joe Raedle/Getty Images)

In this occasional series, we will bring you up to speed on the biggest national security stories of the week.

As the remnants of Hurricane Harvey pull away from Texas into the Mississippi Delta, the U.S. military’s efforts to assist in rescue and relief continues to grow and evolve. Each service now has units involved, and some have been carrying out rescue operations for days.

Here are three commonly asked questions about the U.S. military effort:

Who is involved, and why?
The initial response to the storm and its historic flooding was primarily based on an obvious factor: geography. The Coast Guard, for example, has an air station on the southeastern outskirts of Houston and has been busy with rescue operations from the beginning. The entire Texas National Guard was quickly activated, and other military units from Fort Hood, Tex., to Galveston, where the Marine Corps has a unit of tracked amphibious vehicles, joined the effort.

But the effort is now much broader than that and includes units from across the country. In one example, the amphibious ships USS Kearsarge and USS Oak Hill left port in Norfolk on Thursday. They are expected to steam around Florida into the Gulf of Mexico in coming days.

Additional forces joining the effort have specialized missions. The Air Force has sent about 1,000 airmen and 30 aircraft. They include helicopters, at least one tanker plane and an E-8 JSTARS, a surveillance plane that can gather detailed information about conditions on the ground.

The Navy also has helicopters flying rescue missions and a P-8 Poseidon surveillance plane that is usually deployed for reconnaissance and finding submarines underseas.

Why weren’t more troops sent immediately?
This point has been somewhat contentious, particularly after Russel Honoré, the former commander of the joint task force that responded to Hurricane Katrina, called the response to Harvey “amateur hour.”

In the Pentagon, Defense Secretary Jim Mattis has told his staff to lean forward and be ready to provide whatever is needed, and the services responded by moving some aircraft closer to affected areas before their help was requested by Texas. But the federal law known as Title 10 requires that a state government formally request help before federal active-duty or reserve forces can assist in disaster relief.

Maj. Gen. James Witham, the director of domestic operations for the National Guard Bureau, was asked Tuesday whether the magnitude of the situation was recognized quickly enough by Texas state officials. He responded that it was “debatable” but that even with the disaster planning that is commonly done, it would have been hard to account for the scope of the disaster.

“If you’re looking at an event that only occurs every few hundred years, the planning that would have normally occurred for that probably wasn’t here,” he said. “So, in many cases, the request for assistance, not only for the National Guard but federal forces, may not have been anticipated quick enough. But we’re providing everything we can as quickly as the state asks for it.”

What’s next?
If past is any precedent, the military response could span weeks, if not longer. The deployment of the Oak Hill and the Kearsarge alone indicates that the military is planning to run relief operations for an extended period and will have new bases at sea from which helicopters can fly in supplies and on which Marines and sailors can take a break between shifts.

After Hurricane Sandy in 2012, U.S. troops spent weeks carrying out tasks such as pumping floodwater out of affected areas in New York and New Jersey, in addition to delivering supplies and moving people with aircraft and vehicles.

Military organizations such as the Army Corps of Engineers also will probably be involved in efforts to better prepare Houston for future flooding events.