The massive wildfires burning across California have highlighted long-standing strains on the state’s mutual-aid system, which is designed to quickly rally support from nearby fire agencies to help protect homes and save lives.

From Tuesday through Saturday last week, requests for more than 900 fire engines from commanders around California went unfilled, according to state officials, leaving emergency responders in many communities with far less help than they had hoped for.

During that five-day period, the Carr Fire swept into the outskirts of Redding, destroying hundreds of homes; the Ferguson Fire continued its march toward Yosemite, prompting a rare closure of parts of the national park; and multiple blazes erupted in Mendocino and Lake counties, burning more than 90,000 acres so far.

“People have to be comfortable sending resources and still being able to take care of their day-to-day 911 calls at home,” said California Fire and Rescue Chief Kim Zagaris, who oversees the state’s fire mutual-aid program. But “if everybody decided they aren’t going to send anyone help, then we’d have a lot more houses on the ground and a lot more issues to deal with.”

California’s mutual-aid system is designed to move firefighters and equipment into disaster zones when local first responders find themselves overwhelmed. Calls for assistance first go to nearby counties, then to other agencies across the state. Many experts consider the system one of the best in the country, but there are signs that it’s faltering.

As wildfires have become more intense and dangerous, the number of unfilled requests for fire engines and water tenders sharply increased, from just 134 in the 2012 fire season to 3,029 in the 2016 season. Officials at the state Office of Emergency Services, which runs the program, did not provide the total number of calls for aid in that time period.

The shortage of resources was acutely felt in Wine Country last October, when commanders requested hundreds of engines during the early hours of the two most devastating blazes: the Tubbs and Atlas fires. Less than half of those sought would arrive.

That initially left local firefighters largely on their own to combat a situation that would ultimately require an international effort to extinguish, with thousands of firefighters eventually converging on the area.

Experts have attributed the mutual-aid system’s growing gap between supply and demand to dwindling resources, local budget cuts and a ballooning number of calls for service.

Meanwhile, the increasing severity of wildfires has created a troubling feedback loop: As fires become more damaging, local agencies are hesitant to send help to other jurisdictions, fearing they’ll be caught flat-footed should a disaster unfold in their own backyard.

“This issue we’re dealing with has been going on the last five years,” Zagaris said, referring to the lack of mutual aid during large wildfires. “The size of the fires, the magnitude of the fires, the complexities of the fires are really taxing us.”

When a series of fires start in short succession across the state, as happened in July, it can also create logistical problems, as crews find themselves jumping from one incident to another.

Last week, firefighters drove from Northern California to help fight fires in Southern California, only to be turned around shortly after arriving, Zagaris said. Other crews traveled to the conflagration near Redding, and then were rerouted to deal with large fires that had erupted in their own regions.

When two blazes ignited in Mendocino and Lake counties Friday, California’s firefighting resources had already largely been depleted, state officials said, forcing local commanders to work with less than they would have liked.

“As far as stretched thin, we were and we are,” said Scott McLean, spokesman for the California Department of Forestry and Fire Protection, or Cal Fire. “You only have a certain amount of resources in the state of California.”

Help has begun to arrive from as far away as Maine and Florida. State officials are considering asking for ground troops from the Marines or Army to help clear brush to stop fires from spreading.

Commanders at the major fires that started last week in Northern California were not immediately available for comment. A spokeswoman said responders did not have issues getting help when the Ferguson Fire near Yosemite broke out in mid-July.

While no amount of mutual aid can stop powerful, wind-driven wildfires — like those that hit Redding this year and Santa Rosa last year — it can help save homes and get people evacuated. Early assistance is crucial, said Carroll Wills, spokesman for the California Professional Firefighters, a lobbying group that represents thousands of firefighters.

“Obviously it takes time to roll engines from other parts of the state, but it’s in those first few hours when the people on the line feel it,” he said. “In those early hours, when something really nasty is happening, they’re alone.”

The Legislature recently approved the budgeting of $50 million that can be used to send firefighters to high-risk areas before blazes break out and when conditions are considered prime for wildfires. The money, which became available this summer, will also fund upgrades to fire engines and other equipment.

The budget request was in response to the growing number of unfilled calls for aid, which peaked in 2017 at 11,000 — nearly one-third of all requests, according to state figures. The funds are intended to put responders in a position to stop fires before they can grow and make the system more proactive.

One of the main lawmakers who pushed for the improvements, state Sen. Mike McGuire, D-Healdsburg, did not know whether the money was used in the recent Northern California blazes. Despite the improvements, he said, more needs to be done.

“Out mutual-aid system, built off a neighbor-helping-neighbor approach, was established in the 1950s; it’s underfunded, antiquated and we have to make improvements,” McGuire said. “This is not hyperbole — we desperately need more resources.”

San Francisco Chronicle

staff writer Kurtis Alexander contributed to this report

Joaquin Palomino is a San Francisco Chronicle staff writer. Email: jpalomino@sfchronicle.com Twitter: @JoaquinPalomino