Natural uranium is made up of two different isotopes: About 99.3 percent of it is uranium-238, and 0.7 percent is uranium-235. Making uranium suitable for nuclear weapons requires enrichment. This dramatically increases the concentration of uranium-235 atoms, which can be split in the process known as fission.

Why is this important for nonscientists to understand? Because up until last week, Iran had claimed that its nuclear program was meant to enrich uranium to a concentration of 3.5 percent uranium-235, which is the level necessary for nuclear power generation. In early February, however, Iran announced it was moving to enrich uranium to the 19.75 percent level required for "medical use purposes."

A few days later, on this month's 31st anniversary of the Islamic revolution, President Mahmoud Ahmadinejad proclaimed that Iran had become a nuclear state by reaching 19.75 percent enrichment. Former Israeli ambassador to the United Nations Dore Gold put this in layman's terms when he said Iran's nuclear program is "rounding third and heading for home."

Even the United Nations' International Atomic Energy Agency announced that Iran appears to be developing "a nuclear payload for a missile."

No kidding?

It gets worse. Secretary of State Hillary Clinton announced last week that Iran's Islamic Revolutionary Guard Corps, the keeper of the nuclear program, was taking control of the government away from Shiite clerics. One clue: The annual anniversary celebrating the Islamic revolution had been transformed into a display of military force that suffocated any attempt at dissent.

Like Ahmadinejad, the Revolutionary Guards' leaders were handpicked by the radical Shiite clerics, and most are theological hard-liners. Unlike the majority of Iranians and even the mullahs, though, these military leaders are driven to spread terror and provoke conflicts for geopolitical and theological purposes.

While developing their nuclear capabilities, the Revolutionary Guards are also pursuing defensive weapons, mostly S-300 antiaircraft missiles from Russia, to protect their facilities. In spite of recent delays, Moscow has insisted it will deliver five batteries of the weapons to Iran, which would enable it to shoot down aircraft and cruise missiles.

So what would be the consequences of a Revolutionary Guard-controlled, nuclear Iran?

Iran is the world's No. 1 state sponsor of terrorism, supporting terrorist groups such as Hezbollah and Hamas with money and weapons. Think dirty bomb.

Iran's nuclear arsenal could also serve as a defense umbrella for terrorist attacks. After 9/11, we were able to identify the state sponsor of al-Qaeda (Taliban-ruled Afghanistan), defeat the regime, and eliminate its ability to further support terrorism. Now imagine if a terrorist group attacked the United States and we traced the planning and support to a nuclear Iran.

We only have to remember the days of the Cold War, when the Soviets supported attacks on American interests. Bottom line: Our options would be limited.

Furthermore, it is one thing to deal with an atheistic nuclear regime that cares only about this life. But are we prepared to engage in nuclear brinkmanship with theocrats who are preoccupied with fulfilling apocalyptic prophecy and realizing rewards in the hereafter?

Then there's Iraq. The Revolutionary Guards' Quds Force has been arming Shiite radicals there for years. And Iran has made no secret of its desire to influence the government in Baghdad. Already, with the United States' announced withdrawal, Iran's influence has grown dramatically in Shiite circles. A nuclear Iran without a U.S. presence in Iraq means the latter could become an Iranian client state.

Then there's the almost certain arms race in the region. Chiefly Sunni countries such as Saudi Arabia, Turkey, and the wealthy Gulf states would not sit by quietly as a radical Shiite regime asserts its influence in the Middle East.

Finally, there are the Iranian regime's repeated threats to wipe out Israel -- something to consider as President Obama continues to reach out to Tehran's increasingly irrelevant civilian leaders, oppose meaningful sanctions, ignore the internal opposition, and resist military strikes on enrichment facilities.

Rick Santorum is a Senior Fellow at the Ethics and Public Policy Center and former U.S. Senator (R-PA).