If a person is fortunate enough to live into his ninth decade, life often turns toward quiet reflection, relaxation and the comforts of family and community. Not for the 85-year-old Ayatollah Ali Khamenei. The sunset years of Iran’s supreme leader have been defined by a series of daunting challenges: regional humiliations, domestic uprisings, the looming threat of war with Israel and a pivotal decision on whether to pursue nuclear weapons — a choice with profound implications for his political legacy and the country he has ruled for 35 years.

In the past 100 days, Mr. Khamenei has endured devastating losses. Israel struck decisive blows against Iran’s so-called axis of resistance, including the assassination of the Hamas leaders Ismail Haniyeh in Tehran and Yahya Sinwar in Gaza and the elimination of Mr. Khamenei’s most important ally, the Lebanese Hezbollah leader, Hassan Nasrallah. Additionally, on Oct. 16, the United States sent B-2 stealth bombers — $2-billion-dollar aircraft capable of delivering 30,000-pound bunker-busting bombs — to destroy weapons depots in Yemen linked to Iran’s Houthi allies. It was another blow to Iran’s proxy armies, and a clear signal to Tehran that its underground nuclear sites are within reach.

Significant domestic setbacks preceded these embarrassments. America’s assassination of the top military commander General Qassem Soleimani in January 2020, followed by Israel’s killing of the nuclear scientist Mohsen Fakhrizadeh later that year, underscored the regime’s vulnerabilities. When Mr. Khamenei’s protégé and potential successor, President Ebrahim Raisi, died in a helicopter crash in May, many Iranians suspected internal or external sabotage. Years of mismanagement, plunder and repression, all from a theocratic pedestal, have fueled widespread governmental mistrust and dissent. The national uprising in 2022 and 2023 — known as the “Women, Life, Freedom” protests — required six months and over 20,000 arrests to extinguish. Even absent strict enforcement from the Biden administration, Iran remains among the most sanctioned countries in the world.

In short, Mr. Khamenei has spent the autumn of his life violently repressing a population that wants to unseat him while simultaneously engaging in a sophisticated military and financial conflict with Israel and the United States. Now, on the cusp of a major military attack by Israel, a nuclear power, the supreme leader faces a critical choice: whether to pursue nuclear weapons.

Until now, Iran has maintained a strategy of nuclear ambiguity, attempting to deter its adversaries by staying just short of developing a nuclear weapon without the severe economic and diplomatic penalties associated with one. Although Iranian officials long emphasized that Mr. Khamenei issued a fatwa forbidding nuclear weapons under Islamic law, they now openly acknowledge their capacity to build such weapons if they choose, echoing U.S. intelligence assessments. Iran’s domestic media estimates the total cost of the country’s nuclear program — including sunk expenditures, lost energy revenue and foreign investment due to sanctions — can be measured in the hundreds of billions of dollars, yet it contributes a mere 1 percent to Iran’s energy needs.

Perhaps the most important role that Iran’s nuclear program has served for the last two decades is diverting attention away from its cultivation of missiles, drones and potent regional proxies. As a senior Gulf official once told me about the U.S.-led strategy toward Iran: “We spend all of our time trying to prevent them from acquiring a weapon they will never use, while neglecting the weapons they and their proxies use against us every single day.”