MEXICO CITY — For 75 years, through tensions and disputes over immigration, narcotrafficking and trade, Mexico and the United States have sent each other billions of gallons of water annually to irrigate farms along the border under a treaty signed during World War II.

But today, the 1944 agreement is facing increasingly violent opposition in drought-parched Chihuahua state, where protesters have seized control of a major dam to dramatize the plight of farmers whose cotton, tomato and pecan crops, they say, depend on the water that’s being sent north.

Unrest has simmered for months over U.S. demands that Mexico pay off a shortfall of more than 100 billion gallons by Oct. 24 to meet its five-year water delivery quota. Local farmers say the extra payments are emptying dams that store water they need.

The crisis erupted in violence this month when about 2,000 protesters swarmed over La Boquilla dam on the Conchas River and a national guard unit was sent in to stop them. One woman was shot dead in the chaotic confrontation last week, but about 200 protesters, armed with sticks and rocks, were able to repel the security forces and retain control of the century-old hydroelectric facility.

“We tried to have a dialogue, but nobody listened to us,” said Guerrero Carillo, a local farm leader, reached by telephone at the dam. “There are thousands of farmers desperate for water. We have had no rain in months and there will be nothing left for the spring crops.

“We are prepared to stay here and defend our rights to this water.”

Since the confrontation on Tuesday, tensions have escalated, with generators at the dam set on fire, causing a power blackout. Federal security officials accused the protesters of “sabotage and sedition,” and 500 more guardsmen were sent to the area. But they have not taken any action, and 17 guards are under investigation in the killing of Jessica Estrella Silva Zamarripa, whose family grows pecans.

A second protest broke out Friday about 300 miles away, next to a bridge connecting the border cities of Juarez and El Paso, Tex., Demonstrators decried Silva’s death and demanded that Mexico stop sending its water north.

The treaty requires the United States to send far more water to Mexico than it receives, but those payments flow elsewhere on the 2,000-mile border, far from dry Chihuahua, which provides more than half of Mexico’s share. Every year, Mexico pipes about 114 billion gallons of water north from the Rio Grande and Conchas rivers; the United States sends almost 489 billion gallons south from the Colorado River.

The crisis has sparked a nasty political fight between Mexican President Andrés Manuel López Obrador and opposition politicians ahead of legislative elections next summer. The president, who is committed to paying the entire water debt, has blamed outside forces for fomenting the farmers’ uprising. On Friday, he named 17 current and former opposition party officials he said were working with private businesses to incite the takeover at the dam.

“What we are confronting are the owners of water,” López Obrador told reporters Friday as he displayed the names and faces of the 17 politicians. “There is a clear alliance of politics and economics.”

Several of those he named countered that the president was trying to deflect attention from his decision to accede to U.S. demands that Mexico send a large quantity of water north in a short time, while Chihuahua suffered record-low rainfall through the summer on top of an extended drought.

“The president did not want to have problems with Mr. Trump, so he released the dam water,” said Sen. Gustavo Madero Muñoz, who represents the National Action Party in Chihuahua and was named by López Obrador as a protest conspirator. “He did it in a clumsy and authoritarian way. There were no meetings, no explanation. He didn’t want to accept the blame, so he has invented other guilty parties.”

López Obrador has said that Mexico must uphold its part of the treaty, but he has also expressed concern that the Trump administration might impose tariffs or even close the border if it fails to do so. In July he approved releasing dam water normally stored to irrigate Mexican crops, but said he might appeal to Trump for clemency if the amount falls short.

Sally Spener, the U.S. foreign affairs officer on the International Boundary and Water Commission in El Paso, called the current protests “unfortunate.” She said the Mexican government has “repeatedly stated its intention to meet its treaty obligations” and is attempting to do so. The worry, she said, is that Mexico is “running out of time” to meet the treaty deadline. “It’s an awful lot of water to send.”

With rainfall at 30 percent of normal levels this summer, agricultural officials in Chihuahua have warned that the next planting season could be in danger. Salud Ochoa, a local journalist, said farmers are struggling now; without enough dam water, they fear, they won’t be able to plant at all in the spring.

“The crops people grow here need a lot of water — tomatoes, onions, chiles, cotton, nuts,” Ochoa said. “They have to cover the current season and think ahead to the next one, too. There are three dams here, and with the amount they are sending to the U.S., the water levels are very low. By 2021, it will get worse.”

Mexico’s national water commission says there’s enough water to meet the treaty obligations as well as the needs of Chihuahua growers. In a statement Friday, the commission said all but 11 percent of the water owed to the growers in irrigated districts has already been distributed from La Boquilla dam. Other agricultural areas still rely on rainfall.

But statistics on massive, complicated water flows are hard to pin down. Mexico and the United States disagree on how much Mexico still owes, and rumors have spread in Mexico that the United States has overstated its water contribution. At one point, López Obrador suggested that a United Nations audit could be conducted to determine whether there had been faulty accounting.

On Friday, federal security officials accused protest leaders of cutting back the water flow from La Boquilla to local farms as a means of “pressuring” farmers to join their takeover. But by Saturday, a widening array of supporters had appeared at the site, from businesses donating food to political activists demanding that López Obrador resign.

Carillo, who has remained at the dam since the takeover began, rejected allegations that the protesters were being led by outsiders or were targeting fellow farmers. He described standing up to national guard members who used rubber bullets and tear gas, and said the surge of popular support had “given us courage.”

“We have done this alone,” he said. “No party or politicians have helped us. We want the president to come and see our empty dams. We are fighting for our future.”