The fighting in recent weeks is the latest chapter in a bitter civil war that has raged on-and-off for more than a decade.

“It is clear that in the years when there was supposed peace, the Houthis and their allies in Iran were preparing for war,” said one of the Yemeni officials, who spoke on the condition of anonymity to discuss security issues. “And this is not a war that will affect just Saudi Arabia and Yemen, but the entire world.”

The spiraling conflict is now the most volatile front in the sprawling regional conflagration that was unleashed when the United States and Israel attacked Iran in February, drawing retaliatory strikes by Tehran on Persian Gulf nations and by Hezbollah, the Lebanese militant group, on Israel.

With weeks to go until the U.S. midterm elections, high fuel costs have turned voters against President Donald Trump’s Republican Party, and the White House is trying every lever to bring down costs, including lifting sanctions on Russian diesel exports — so far without success or any change in the polls.

Trump, who is close with Crown Prince Mohammed bin Salman, the de facto ruler of Saudi Arabia, has been reluctant to get involved militarily in the war against the Houthis. A U.S. bombing campaign against the militants last year caused high casualties but ultimately did not stop the conflict.