(CNN) -- A security alert in New York's subways. A major speech by President Bush. A purported letter from one of the world's top terrorists to another. There's a common thread to all these stories -- the Iraqi insurgency.The dubious tip about the alleged threat in New York came from Iraq; the president's speech about Islamic terrorism was provoked by the fighting in Iraq. And the purported letter from al Qaeda's number two, Ayman al-Zawahiri, was addressed to Abu Musab al-Zarqawi, the leading foreign fighter in Iraq, and now the head of al Qaeda there.Earlier this year, my colleague Nic Robertson and I spent time investigating the insurgency and the men who, we discovered, like to refer to themselves as the Iraqi Resistance.With the constitutional referendum in Iraq and Saddam Hussein's trial looming, here is an Iraq Insurgency 101.How many insurgents are there?Up to 200,000, according to U.S. military intelligence sources. That seems astonishingly high, given official estimates of 5,000 to 25,000. The sources say their 200,000 figure includes 25,000-30,000 actual fighters while the rest are active and passive supporters, including fund-raisers, lookouts and even family members.But the numbers are growing.One U.S. military official --- looking back at the past year --- put it this way: "If someone would have told you that we would have killed 10,000, wounded 10,000 and imprisoned 32,000, you would have thought we would have won."The problem, he adds, is "the insurgency is self-regenerating."How and when did the insurgency begin?A former Iraqi general, who now leads an insurgent group in Baghdad, told us he and others were planning resistance even before the U.S. invasion. The general, who goes by the alias Abu Omar, said: "Six months before the occupation, we started training and exercising resisting the American army in small groups."U.S. intelligence and military officials say many of the groups now making up the resistance were factions who benefited under Saddam Hussein, including army officers, a paramilitary group called the Saddam Fedayeen, and various Sunni tribal groups. They had their own networks for communications and even for financing to get around U.N. sanctions before the fall of Saddam.Does that mean the insurgency was inevitable?"The insurgency was not inevitable," said former Maj. Gen. James "Spider" Marks, who headed intelligence for U.S. ground forces in Iraq before and after the invasion. He and the Iraqi ex-general, Abu Omar, both say that what might have made the difference was the decision by U.S. administrator Paul Bremer in late May 2003 to disband the Iraqi army, throwing hundreds of thousands, mostly Sunnis, out of work.At about the same time in May 2003 -- the month after the regime was overthrown -- Saddam Hussein's deputy Izzat Ibrahim al-Duri held a meeting in a car in Baghdad with four other top Saddam loyalists, where they decided to activate an insurgency, according to U.S. intelligence sources.Who are the insurgents?A mix of people and networks, according to both the insurgents and U.S. sources. Most are Sunni Muslims -- a minority in Iraq but who held pre-eminent positions under Saddam.There are also a growing number of disappointed and disillusioned tribesmen from the Sunni triangle, the area that includes the cities of Ramadi and Falluja, where many powerful local tribal sheiks once thought they could work with the invading Americans."The first day the American forces entered the province, we went and negotiated with great hopes and expectations due to what we had heard about America and its democracy," says Sheik Zeidan, who controlled 20,000 men. The U.S. military kicked Zeidan out of the country last year and has refused to negotiate with or pay court to many of these tribal leaders.Then there are the foreign fighters, most notably Abu Musab al-Zarqawi.How many foreign fighters are in Iraq?Surprisingly few, numbering perhaps 500 to 1,500. They carry out the most spectacular and bloody attacks, including the suicide missions that have, at times, killed more than a 100 people in a single day and the beheadings, such as that of American Nicholas Berg. There are four times as many people going after Zarqawi and his network, than looking after the rest of Iraq. -- Senior U.S. military official Al-Zarqawi and his followers, who call themselves al Qaeda in Iraq, are as sophisticated with their public relations as they are brutal with their attacks. They videotape their actions and executions, posting them to Web sites.In that recent speech about Iraq, President Bush mentioned al-Zarqawi almost as many times as he did Osama bin Laden. But all that attention may also be tilting the military effort against the insurgency. "There are four times as many people going after Zarqawi and his network, than looking after the rest of Iraq," says one senior U.S. military official.What exactly is the al Qaeda connection?Al-Zarqawi and bin Laden may have crossed paths in Afghanistan in earlier years, but it wasn't until last year that al-Zarqawi publicly swore allegiance to bin Laden. For his part, bin Laden soon followed by naming al-Zarqawi his deputy for Iraq. It is an arrangement that pays off for both men, say al Qaeda experts.Who are the insurgents targeting?The U.S. military was the initial and primary target for the Iraqi insurgents. But they've changed their focus to Iraqis they see as collaborating with the occupiers, which is why Iraqi military and police forces and, now, teachers have been targeted.And al-Zarqawi's group, especially, has targeted the Shiite Muslim majority, with a series of vicious attacks.What should we most be worried about?Three things. First, even though numbers are hard to come by, you can measure the strength of the insurgency by the number of attacks. Although there was a lull around the time of the Iraq election last January, the attacks have increased since then. "I believe that resistance is not confined to certain persons or organizations. Resistance is now the prevailing culture in Iraq," Sheik Zeidan says. The youth wants immediate results therefore he will join al Qaeda to inflict most harm against the enemy. -- Abu Omar, insurgent leader Second, and perhaps more worrying, is that al Qaeda no longer seems to be made up of foreign fighters. Abu Omar, the insurgent leader, says he welcomes al Qaeda fighters, but stresses that the resistance is Iraqi.Yet, he says, the situation may be spinning out of control as younger Iraqis join al Qaeda, attracted by its ability to carry out those daring attacks. "Those who like to inflict the most harm on the Americans prefer to join al Qaeda. The youth wants immediate results therefore he will join al Qaeda to inflict most harm against the enemy."Third, there is the possibility of civil war between the Sunnis and Shiites. A recent posting by al-Zarqawi's group made it clear one aim was "targeting the infidel militias and the [prominent leaders] who represent heresy and atheism among the Shiites."In that letter from al-Zawahiri to al-Zarqawi, al Qaeda's number two admonished al-Zarqawi for attacking the Shiites, or at least being public about it: "Even if we attack the Shiites out of necessity, then why do you announce this matter and make it public?"Nevertheless, civil war remains the biggest fear.Is there a way out?That may be the toughest question of all. U.S. military officials told us the same thing they say they've told their bosses, even the president: ultimately there is no military solution, there has to be a political deal between Sunnis, Shiites and the Kurds.For his part, Abu Omar talks tough, saying: "no negotiations, until we kill the last American soldier." But in the next breath, he adds "if they (the Americans) want to be serious, let it be official and in front of all people."