Deputy Defense Secretary Paul D. Wolfowitz provided the Senate Foreign Relations Committee this week with a rare admission of error. His observations may not be revelatory, but they give Congress an opening to assume more responsibility for what happens next in the war.

Mr. Wolfowitz conceded that the administration had miscalculated the extent and intensity of resistance in Iraq. “We had a plan that assumed we’d have basically more stable security conditions than we’ve encountered,” he said.

Evidence that this mistake was made not only before the war but well after it also can be seen in a quarterly index of the Iraq effort from the Brookings Institution. Under its Security Indicators it lists the monthly totals of insurgents detained or killed. For April, August and December 2003 and April 2004, the rough numbers were 1,000, 1,000, 1,000 and 2,000; the total estimated number of insurgents in Iraq for the final three dates were 5,000, 3,000-5,000 and 5,000, which is to say that capturing or killing insurgents doesn’t seem to reduce the number of insurgents. The relatively small number of foreign “jihadists” has multiplied tenfold to 500 during this period, according to the index.

These numbers highlight the challenge the United States now faces: committed resistance that, as attacks have shown since last fall, is well-armed and well-organized. Senators, Republican and Democrat alike, have been tough on administration officials as the fact of war has failed to meet its advanced billing. But they also need to do more.

Mr. Wolfowitz was before Congress recently to discuss the $25 billion in supplemental aid that appears to be only a downpayment on the money needed to continue to war in Iraq, but “appears” is about the best one can say about it because the request for funding included very few details. Sen. Susan Collins told the deputy defense secretary that Congress would be neglecting its duties if it approved the money without more explanation.

Congress can’t very well refuse to fund the war, but it can insist that the funding be exchanged for a much clearer sense of what the administration intends to do to win it. Congress should also demand that it be given a greater role in decisions and more complete information faster. If the administration hopes to keep congressional support, it will admit that it hadn’t properly assumed that conditions in Washington could deteriorate, too.