By Mike Wooldridge
BBC World Affairs Correspondent, Baghdad
Moqtada Sadr is under pressure to re-engage in the political process
Within days of Moqtada Sadr's politicians returning to Iraq's coalition government after a two-month boycott, more evidence has come to light of the pressure being put on the radical Shia cleric's powerful militia, the Mehdi Army.
The US military said joint Iraqi and American operations had led to 16 "high-level" Mehdi Army militiamen being detained and one commander being killed.
The Americans said the number of Mehdi Army members in detention now stood at over 600, a figure higher than that given by Iraq's Prime Minister, Nouri Maliki, only a week earlier.
But this was said to be a result of 52 operations carried out over a six-week period and, pointedly, the statement giving details of these moves against the Mehdi Army also said there had been 42 raids targeting Sunni extremists.
The statement was headed: "Iraqi security forces and coalition forces combine in balanced approach to security" - foreshadowing the declared approach of the new security plan for the capital that is the centrepiece of President Bush's new Iraq strategy.
The radical cleric has vocal and well organised supporters
There are estimated to be several thousand Mehdi Army members in Baghdad alone and many thousands more around the country.
The Americans accuse the militia of being deeply involved in the sectarian violence that has taken root in Iraq since the attack on an important Shia shrine in the town of Samarra.
A recent Pentagon report said the militia was the largest threat to security.
It is said by some analysts here that the prime minister was originally less persuaded than some of his fellow politicians in the ruling Shia alliance of the potency of the Mehdi Army threat.
Whatever the truth, Mr Maliki put the emphasis on political engagement with Moqtada Sadr, a key political ally.
The issue played out in political developments within the alliance in the closing months of last year.
The US ambassador to Iraq, Zalmay Khalilzad, says he believes Mr Maliki came to feel that political pressure on Moqtada Sadr was not producing results and he "changed the mix" to allow more use of force against the Mehdi Army.
Mr Maliki has given commitments both in public and in private that no group that is involved in violence will be off limits in the new drive to bring security to Baghdad.
Then came the decision by the Sadr movement to return to the political process after a boycott that was in part a protest against the prime minister meeting President Bush, and also to press the government for a timetable for the withdrawal of US troops.
It could signal that Moqtada Sadr wants more leverage with the government by being back inside at a time when the pressure is building on the Mehdi Army.
There are signs that Moqtada Sadr is not seeking a confrontation with US forces after their battles of the past
If so, the issue then is how will his political group use its influence?
Some say Moqtada Sadr might not be averse to action against renegade members of his militia.
The US ambassador welcomed the return of the Sadr political group to the government but he suggested it could also be a tactic to try to keep the militia safe during the current operations against them.
Away from the spotlight of the capital, there are other places where the heat is clearly on the Mehdi Army.
Residents of villages some 25 miles (40 km) north of Baghdad say they have seen US troops carrying out raids targeting the militia - hunting down individual figures.
There is one other factor in the pursuit of the Mehdi Army.
Recent bombings in Baghdad that have killed scores of people and appear to be the work of Sunni insurgents are only likely to reinforce the view of many Shias that, for now, the Mehdi Army are their most realistic protectors.
There are signs that Moqtada Sadr is not seeking a confrontation with US forces after their battles of the past.
But the new drive to pacify Baghdad has, of course, hardly begun.