At the height of the war, in 2007, terrorists claimed 5,425 civilian lives and caused 9,878 injuries. The violence has since declined, but Sunni militants have revived their campaign against Prime Minister Nuri Kamal al-Maliki, a Shiite. A series of attacks on Tuesday killed at least 52 Iraqis, including a Finance Ministry official, and injured 180 more.

These Sunni militias are determined to regain the leverage they had lost in their war against American forces. Suicide bombers have launched several attacks since January, while Al Qaeda in Iraq is regrouping in Anbar Province in western Iraq, adjacent to Syria. On March 19, terrorists killed almost 60 people in several bombings in Shiite neighborhoods in Baghdad. The security situation is now sufficiently fraught that elections will be delayed in Anbar and Nineveh provinces for at least six months.

The costs of the terrorism inspired by the war include much more than the number, however horrifying, of lives lost. The terrorists who have been drawn to Iraq since 2003 and survived have been battle-hardened after fighting the most sophisticated military in history, often working together with former officials from Saddam Hussein’s Baathist regime. They have developed expertise in counterintelligence, gunrunning, forgery and smuggling. Smuggling routes and alliances that moved terrorists and supplies into Iraq during the height of the war, in 2006-7, have been reversed, allowing fighters and supplies to flow into neighboring countries, particularly Syria, now in its third year of civil war.

Al Qaeda in Iraq is now increasingly active abroad. In October 2012, Jordanian authorities detained 11 suspects whose alleged goal was to “kill as many people as possible” and to “bring Amman to its knees.” Al Qaeda in Iraq is also playing an increasingly important role among the Islamists fighting President Bashar al-Assad’s regime in Syria. Of especially grave concern is the movement into Syria of bomb makers and military tacticians. As Iraq’s jihad was for much of the past decade, Syria’s is now becoming the “destination jihad” du jour.

The exacerbation of Sunni-Shiite tensions has contributed to the creation of fighting forces capable of exploiting those tensions throughout the region. Most prominent among the Sunni jihadist groups in Syria is Jabhet al-Nusra, led by a veteran of the Iraq insurgency, Abu Muhammad al-Jawlani. Jabhet al-Nusra is fighting Mr. Assad’s regime, with the aim of establishing an Islamist state in Syria. Although Iraqi Shiites have traditionally opposed Mr. Assad’s regime because of its connections to the Baathist movement represented by Saddam Hussein, they now see in the Syrian uprising the signs of a sectarian civil war, and some are traveling to Syria to support Mr. Assad, a fellow Shiite.