The Senate came into its own yesterday and put to General David Petraeus the central weakness of his case. Senator Joe Biden, a Democrat sceptical of the war and of the US’s attempts to hold Iraq together, challenged Petraeus, the US commander, and Ambassador Ryan Crocker to explain why present military success meant lasting political improvement.

Even if the military tactics have worked, he argued, while Iraq lacks a government that will embrace all factions, then when US forces go home, Iraq will fall apart.

It is hard to argue against that, and Petraeus and Crocker did not do so. In analysis of Iraq, they are towards the upbeat end, but not to the point of implausibility. But although Petraeus’s confidence about military progress will not have disappointed the White House, both were rightly cooler about Iraq’s politics. The Shia-led Government interprets democracy as winner-takes-all, and, so far, the US has failed to persuade it to take less in the cause of national unity.

The two Senate committees yesterday pressed the point home where the sprawling House of Representatives panel on Monday failed, losing direction in effusive compliments to Petraeus. For all the fanciful comparisons with General Dwight Eisenhower (suggesting that Petraeus may eventually be offered political office), it is not going to be easy for anyone to win advancement by association with Iraq.

In contrast, the Senate Armed Services and Foreign Relations committees, always packed with presidential wannabes past and future (now John McCain and Hillary Clinton), challenged Petraeus directly on his optimism and projections of troop numbers.

Petraeus resisted politicisation in his analysis of Iraq’s main problems. Despite being pushed by the White House on to stage on September 11, he shunned the language of President Bush’s “War on Terror”. He acknowledged the threats from Iran and al-Qaeda, but asserted, rightly, that “the fundamental source of the conflict in Iraq is competition among ethnic and sectarian communities for power and resources”.

Crocker was justified in saying that this conflict reflects the sectarian hatred created by the Sunni persecution of the Shia majority under Saddam Hussein. Iraq was undergoing a revolution, he said, “not just a regime change”.

But although criticism of the US’s understanding of Iraq is embedded in that phrase, it was not explicit. Both Petraeus and Crocker glossed over the four years of US occupation, and whether different decisions might have averted the bloodshed, addressing tactical questions only from the date of their own involvement.

Petraeus’s main claim was that the surge of 30,000 US troops this year, taking the total to 160,000, had quelled some violence. There is no need to be as sceptical of that as many are determined to be; the better line of attack, as the Senate showed, is whether it is sustainable.

One weak point in his position is that the killing may have dropped partly because Sunnis and Shias are now so segregated. A second lies in the value he puts on recent help from formerly-hostile Sunni tribes now recoiling from al-Qaeda brutality. It is not cynical to doubt that this represents real support for the US or a Shia-led government.

Similarly, his view that the Iraqi Army was becoming more reliable is plausible. But that throws the question – like all others – back to the political domain, and whether the Iraqi Government is pursuing the US’s vision of a united country or a brutal majority rule.

Crocker had the worse job yesterday, trying to put an upbeat face on that impasse. He was least convincing in saying that Iraq’s leaders “have the will” to do so.

When Petraeus asserted that the 30,000 surge troops could leave by next summer, his presentation became frankly political. Lack of replacements always meant that the surge could not be sustained. Petraeus did little more than restate that fact, with charts. Beyond that, he was evasive, although justifiably so, as a commander not a politician. His final slide – really, a blank sheet of paper dressed up as a graph – showed US troops staying in Iraq, but lacked any dates. Who could disagree?

It is Bush – or Congress, or the next president – who will have to put the dates on that graph. Bush’s attempt to get Petraeus to fill in the blanks necessarily failed. In the US, as in Iraq, the hardest decisions are not military, but political.

Times Recommends

• The ghost of Robin Cook haunts Chilcot’s feast
• The truth shouldn’t need oaths
• Violence: let’s separate the men from the boys