Iran is moving aggressively to gain influence in our hemisphere, and left-wing, anti-American leaders in Latin America are rolling out the red carpet for President Ahmadinejad. As this Marxist-Islamist entente grows, it presents a clear danger to American security and makes the need for action against Iran’s nuclear program even more pressing. The latest development in the alliance came this weekend, when Mr. Ahmadinejad visited Latin America for the second time in four months. In Venezuela, he was received at the airport by President Chavez. “Welcome to Venezuela, where Iran is beloved,” the Caracas daily El Universal quoted Mr. Chavez as saying. “We give welcome to a distinguished leader, the leader of a heroic people and of a revolution kindred to the Venezuelan revolution: the Islamic revolution.”

The two retired to the presidential palace, Miraflores, to sign off, El Universal reported, on at least 29 memorandums and letters of intent. They also reiterated that they would push for OPEC production cuts, and, the Associated Press reported, they announced plans for a $2 billion joint fund to finance projects that would thwart American domination. “This fund, my brother, will become a mechanism for liberation,” Mr. Ahmadinejad said to Mr. Chavez during the announcement.

The diplomatic footsie continued the next day in Nicaragua, where, according to Iran’s state-owned Islamic Republic News Agency, Mr. Ahmadinejad was awarded two state medals by Nicaragua’s newly inaugurated president and America’s old foe, Daniel Ortega.

Among the agreements signed was one to establish diplomatic relations and open embassies in Managua and Tehran, noted El Nuevo Diario in Managua. Later, the two visited a Nicaraguan slum together. Mr. Ahmadinejad called Mr. Ortega a “symbol of justice,” and Mr. Ortega promised to “fight for … the defense of our sovereignty.” Yesterday the AP reported that Mr. Ahmadinejad was present at the inauguration of Ecuador’s new left-wing president, Rafael Correa, who has promised to shut down an American airbase there.

Until recently, Mr. Chavez and his fellow Latin American leftists were an annoyance for advocates of free trade and good governance. Now that they’ve allied themselves with Islamic extremists, however, they’ve become a great deal more dangerous. As we learned during the Cuban missile crisis, allowing enemies who possess nuclear weapons to set up forward operating bases within range of our cities is not an option — a point, by the way, that was understood by both Democrats, such as President Kennedy, and Republicans, such as Richard Nixon whom he defeated for the White House.

President Reagan understood all this, which is why during the 1980s he fought so hard against Mr. Ortega’s Soviet-backed camarilla and raided Grenada. That the left makes common cause with the Islamists is one of the bizarre facts of modern geopolitics. The only thing Marxists like Messrs. Chavez, Ortega, and Correa have in common today with the likes of Mr. Ahmadinejad is a hatred of America. That is the foundation on which the Marxist-Islamist scheme is formed and the motivation behind any actions it carries out.

Although economic in nature, this entente will eventually become a military one. This has been underscored in the last several years by Venezuela’s billions of dollars in arms purchases, which included both small arms and more serious weapons like the Sukhoi Su-30 Russian fighter jets and Russian-made attack helicopters. There is also increasing troop movement among the left-wing Latin American states. The military ambitions of Messrs. Chavez, Ortega, and Ahmadinejad spell trouble.

Blocking this Islamist maneuvering in Latin America needs to be put at the top of the priorities for American foreign policy in the coming months. The existence of the Marxist-Islamist scheme also increases the importance of putting a definitive end to Iran’s nuclear program. It is no longer a strategic matter of protecting our ally Israel or heading off a Middle Eastern arms race. The threat posed by a nuclear Iran will be to our own cities — Miami, Houston, Tucson — and it will come from Iranian missiles based in the jungles of Nicaragua and on the plains of Venezuela.