When millions of American citizens were left without drinking water or electricity after Hurricane Maria demolished much of Puerto Rico, thousands of U.S. servicemembers descended on the island to assist.

Within weeks, upward of 10,000 troops were in Puerto Rico or just offshore providing much-needed humanitarian relief. Sailors aboard a Navy hospital ship assisted patients after 58 of the island's 69 hospitals were left without fuel or power. Other troops conducted medical evacuations and delivered relief supplies to hard-hit areas.

Assisting Puerto Ricans in the wake of such a devastating storm proved challenging. The Trump administration faced backlash for what some called a slow response. Reaching the stranded and delivering supplies was difficult, though. Roads and other infrastructure were washed away following the Sept. 20 Category 4 storm, and many first-responders were themselves storm victims.

By early October, an Army three-star general was on the ground to lead relief efforts. The number of U.S. troops there more than doubled to about 11,000 after the scope of the devastation became clear.

Maria's wrath also came on the heels of two other destructive hurricanes: Harvey and Irma. Those storms prompted their own massive military responses in places like Florida, Louisiana, and Texas.

Here's how tens of thousands of servicemembers stepped up to help in the wake of back-to-back disasters.

A vital recon mission

Before Harvey and Irma hit, NOAA Cmdr. Scott Price and his crew were flying high above the Caribbean - and straight into the deadly storms.

Flying into hurricanes feels a lot like a roller coaster, says Price, a former naval aviator who flies the WP-3D Orion for NOAA. “It is not uncommon in these storms to go up or down 1,000 or 2,000 feet, and it can happen pretty quickly.”

NOAA's National Hurricane Center was tracking Harvey and Irma long before they became the storms everyone was talking about, Price says. More than a week before Harvey's late-August landfall, which brought historic flooding to heavily populated areas like Houston, Price deployed to the Virgin Islands to fly into the storm. Days later, he and his crew took off from Barbados to fly into Irma - one of the Atlantic's strongest storms on record.

Back on the ground, servicemembers like Coast Guard Capt. Tony Hahn closely monitored data collected on those risky flights. “NOAA and the National Weather Service did a fantastic job forecasting the path and the stagnation over Houston,” says Hahn, commander of Coast Guard Sector Corpus Christi and a MOAA member.

Based on those forecasts, the Coast Guard braced for the worst-case scenario. When Harvey made landfall, dumping more than 50 inches of rain in some areas, members of the Army, Navy, Marine Corps, Air Force, and Public Health Service flocked to the region, joining Coast Guard members in a massive humanitarian mission.

Before their work was complete, some were told to brace for Irma, Jose, and Maria as they careened toward the U.S. and its territories.

“Everyone came with a spirit of service,” Hahn says. “They showed up and asked, 'How can I get the job done?' There was some incredible bravery.”

Reaching the stranded

Since the Coast Guard falls under the Department of Homeland Security, Texas-based coasties were some of the first carrying out search-and-rescue missions in Harvey's wake.

Hahn's sector prepares for hurricanes regularly. With the storm approaching Texas, coasties pre-staged boats and aircraft in areas where they wouldn't get damaged. Coast Guard family members also were ordered to evacuate so servicemembers wouldn't worry about their safety while on duty.

Harvey still was wreaking havoc over the area when multiple mariners reported they were in distress. Hahn had to carefully assess conditions before sending coasties into the fray.

“For me, that was the biggest challenge,” Hahn says. “The stress of … knowing they were in a tough spot while the wind conditions were not conducive to sending aircraft out.”

By daybreak, the winds had subsided, and Hahn's team rescued 19 people. That launched the start of a massive rescue mission for the Coast Guard. Within two weeks, the service saved 11,022 people and 1,384 pets, according to Coast Guard data.

Coasties from as far away as Alaska, Guam, and Hawaii would join their efforts. Members of the Air National Guard from Alaska, California, Connecticut, and Oregon also assisted. Marine Corps Capt. Paul Clarke, a combat engineer officer with Marine Wing Support Squadron 473, was about 250 miles north of Houston in Fort Worth when Harvey made landfall. Tapping active duty and reserve units for domestic missions is rare, and they only are allowed to assist if a request comes from a governor or the Federal Emergency Management Agency.

It took about four days for Clarke's unit to get the call for help, but they'd been preparing long before that.

“We were very proactive,” he says.

Twenty-one members of his unit headed to southeast Texas, where they teamed with 14th Marine Regiment to form a special-purpose Marine air-ground task force.

Clarke says: “Being a Texan myself, seeing the human suffering and devastation was disheartening. It was a great honor to participate in this humanitarian operation where I got to help fellow Texans directly.”

Equipped with amphibious assault vehicles and 7-ton trucks, the Marines - along with members of the Army National Guard - pushed east of Houston into severely flooded areas like Orange and Beaumont. The troops reached those trapped by rising waters and delivered food, water, and toiletries.

“A lot of the people … said the Marines were the very first military responders they saw since Hurricane Harvey hit,” Clarke says. “Our motor-transport truck tires are about 55 inches high, so they could traverse these [flooded] roadways.”

Opening commerce routes

Within days of Harvey hitting, Clarke and Hahn had to allocate resources in preparation for Irma's wrath. Marines, coasties, and other troops were again assisting Americans, this time primarily in Florida.

Saving lives in the wake of the storms was the No. 1 priority, Hahn says. But the Coast Guard and Army Corps of Engineers also were tapped with another crucial mission: reopening vital commerce routes in and around Texas.

“Corpus Christi, Houston, and Beaumont are [some] of the largest petrochemical ports in the whole nation,” Hahn says. “That has impacts on our economy, as our refineries rely on materials coming in by ship to keep them going. We were doing everything we could to get the waterway open without rushing and making a mistake that could cause more problems down the line.”

Reopening industrial ports is a team effort, Hahn adds. The Army Corps of Engineers first had to survey for obstructions, in case vessels sank during the storm or shipping containers had washed out to sea. They also checked the depth of the channel, in case the force of the hurricane changed the seafloor.

Once that work was complete, the Coast Guard had to get ship-channel buoys and markers back in place so pilots and mariners could find their way through the channels. With most of the fixed aids marking the intercostal waterways destroyed, they put up temporary markers until they can replace the telephone pole-like permanent structures, which must be drilled back into the seafloor.

They then were able to open the ports back up during daylight hours to one-way traffic. Additional repairs were needed to resume normal operations, but getting some traffic moving, even with restrictions, was vital to the national economy.

“About $4.5 trillion in commerce flows through the marine highways and all through the maritime infrastructure in the United States, and the Coast Guard just has a huge role in that,” Hahn says.

Providing emergency care

Other vital infrastructure damaged in the storms included medical facilities. At one point during Harvey, 45 hospitals, dialysis facilities, and other care centers were closed due to flooding.

That's when members of the Public Health Service's Commissioned Corps swooped in to help. More than 550 of the Commissioned Corps' officers were deployed or placed on alert status for rapid response to Harvey, while another 150 were on call to deal with Irma, says Tara Broido, a Public Health Service spokeswoman. The officers not only provided direct care to patients but also moved them from locations impacted by the storm and served as liaisons to local partners.

“Since Harvey, health care professionals … are the most in-demand resource,” Broido says, with the most in-demand supplies being food and water. “The [operational tempo] is fast-paced. Most Corps officers are on two-week deployments and are working 12-hour shifts.”

The Public Health Service stood up four 250-bed federal medical stations after Harvey. The young, elderly, and those with compromised immune systems or physical disabilities are at the most risk of medical emergencies after any disaster, Broido says. Top concerns include managing conditions like diabetes as well as the psychological consequences of surviving a natural disaster.

“The most common ailments seen by Corps officers are related to the trauma of having survived the hurricane,” she says. “The Corps' behavioral health providers are there to help those ... dealing with having to rebuild their lives after losing their homes, possessions, and more.”

Thousands of servicemembers at sea also assisted patients after the hurricanes. Marines and sailors aboard the Navy's amphibious assault ships USS Wasp (LHD-1) and USS Kearsarge (LHD-3) and dock landing ship USS Oak Hill (LSD-51) transferred noncritical care patients in the Virgin Islands after Hurricane Irma struck there. And as of late September, the Kearsarge Amphibious Ready Group, which includes sailors and Marines, had conducted at least eight medical evacuations and 123 airlifts, Pentagon spokesman Army Col. Rob Manning says. The troops delivered 22,000 pounds of relief supplies and other cargo to Puerto Rico and the Virgin Islands.

Preparing for the worst

Hahn credits the military's ability to respond to these natural disasters to realistic training scenarios that helped them brace for the worst. Requiring troops to train as they fight is crucial to getting the response right.

“We prepared our team, we prepared our families, and that allowed us to execute this mission,” Hahn says.

Clarke agrees. His Marines were ready to head into some of the hardest-hit areas because of their training. Responding to a domestic disaster means teaming with partners Marines don't always work with, like the Red Cross. But they learned to speak each other's language to get the job done.

Hahn also credits strong leadership from his superiors. They were given good guidance and feedback, but weren't micromanaged.

Having the services' responses play out in such a high-profile way, with rescues being televised live on nightly newscasts, also helps remind the public of what their military is capable of in times of crises.

“When the U.S. at large gets to see how nimble and effective we can be, they understand our value proposition,” Hahn says. “The capabilities we bring to bear are fantastic. I'm proud to be in the Coast Guard, and I'm proud when the public gets to see that.”