On the surface, the march in Baghdad today organized by the radical Shiite cleric Moqtdada al-Sadr was to protest Israel's war in Lebanon, as Iraqi Shiites feel more naturally attuned to the Shiite Hezbollah forces than many of the Sunni Arabs around the Middle East.
But Sadr's secondary motive in staging the march -- which he himself did not even attend -- was to show his power to mobilize large crowds of his supporters, mostly angry young men, on very short notice.
It is no coincidence that this march, one of the biggest Baghdad has seen since the U.S. invasion in March 2003, happened just as the United States announced it was bringing more troops into Baghdad to confront the sectarian violence there.
It has already become clear that Sadr's Mahdi army militia, accused of harboring death squads and targeting Sunnis throughout Baghdad, will be one of the principal targets of the American forces. In the last few weeks, U.S. troops, along with British troops in the south, have staged carefully planned raids against leaders of Sadr's militia.
Today's march was Sadr's way of saying he was not going to let the United States push him around without putting up a fight.
Sadr has taken on the Americans once already, in the spring and summer of 2004, in uprisings centered on the southern cities of Najaf and Kufa.
The Americans methodically surrounded him and pinned his men down. He lost hundreds of fighters against the much better armed U.S. military, and ultimately gave up his military challenge to the United States.
At the time, many people thought Sadr was finished as a major player in Iraq, since the fighting around the holy Shiite sites in Najaf and Kufa was deeply unpopular with many of the more conservative Shiites.
Sadr withdrew and rebuilt his forces, particularly in the sprawling east Baghdad slum of Sadr City, named after his father. His network has followed some of the tactics of Hezbollah, providing welfare for the poorest citizens to garner their loyalty.
This time around, Sadr holds stronger cards. His party won 30 seats in parliament in this year's elections, and so he can apply political pressure as well as military pressure. Given that all sides agree that the violence in Iraq cannot be stopped without a political agreement between Sunnis and Shiites, Sadr has to be part of the negotiations.
And if he doesn't like the way the talks are going, he is not shy of demonstrating his power to mobilize people on the streets.
That could be bad news for the American troops coming back into Baghdad, who could find themselves forced into a second showdown with Sadr.
With today's protest march, Sadr was really saying that he is not afraid of round two.