WASHINGTON -- Come September, America might slip closer toward a Weimar moment. It would be milder than the original but significantly disagreeable.

After the First World War, politics in Germany's new Weimar Republic were poisoned by the belief that the army had been poised for victory in 1918 and that one more surge could have turned the tide. Many Germans bitterly concluded that the political class, having lost its nerve and will to win, capitulated. The fact that fanciful analysis fed this rancor did not diminish its power.

The Weimar Republic was fragile; America's domestic tranquility is not. Still, remember the bitterness stirred by the accusatory question "Who lost China?" and corrosive suspicions that the fruits of victory in Europe had been squandered by Americans of bad character or bad motives at Yalta.

So, consider this: When Gen. David Petraeus delivers his September report on the war, his Washington audience will include two militant factions. Perhaps nothing he can responsibly say will sway either, so September will reinforce animosities.

One faction -- essentially, congressional Democrats -- is heavily invested in the belief, fervently held by the party's base of donors and activists, that prolonging U.S. involvement can have no benefit commensurate with the costs. The war, this faction says, is lost because even its repeatedly and radically revised objective -- a stable society under a tolerable regime -- is beyond America's military capacity and nation-building competence, and is politically impossible given the limits of American patience.

The other faction, equal in anger and certitude, argues, not for the first time (remember the transfer of sovereignty to Iraq, Iraqi voters' purple fingers, the Iraqi constitution, the killing of Saddam's sons, the capture of Saddam, the killing of Zarqawi, etc.), that the tide has turned. How febrile is this faction? Recently it became euphoric because of a New York Times column by two Brookings Institution scholars, who reported:

"We are finally getting somewhere" ("at least in military terms"), the troops' "morale is high," "civilian fatality rates are down roughly a third since the surge began" and there is "the potential to produce not necessarily 'victory'" but "sustainable stability."

But the scholars also said:

"The situation in Iraq remains grave," fatalities "remain very high," "the dependability of Iraqi security forces over the long term remains a major question mark," "the Iraqi National Police" are "mostly a disaster," "Iraqi politicians of all stripes continue to dawdle and maneuver for position," it is unclear how much longer we can "wear down our forces in this mission" or how much longer Americans should "keep fighting and dying to build a new Iraq while Iraqi leaders fail to do their part," and "once we begin to downsize, important communities may not feel committed to the status quo, and Iraqi security forces may splinter along ethnic and religious lines."

The rapturous reception of that column by one faction was evidence of the one thing both factions share -- a powerful will to believe, or disbelieve, as their serenity requires. Consider the following from the war-is-irretrievable faction:

Rep. James Clyburn of South Carolina, House majority whip, recently said it would be "a real big problem for us" -- Democrats -- if Petraeus reports substantial progress. Rep. Nancy Boyda, a Kansas Democrat, recently found reports of progress unendurable. She left a hearing of the Armed Services Committee because retired Gen. Jack Keane was saying things Boyda thinks might "further divide this country," such as that Iraq's "schools are open. The markets are teeming with people." Boyda explained: "There is only so much you can take until we in fact had to leave the room for a while ... after so much frustration of having to listen to what we listened to."

In the other faction, there still are those so impervious to experience that they continue to refer to Syria as "lower-hanging fruit." Such metaphors bewitch minds. Low-hanging fruit is plucked, then eaten. What does one nation do when it plucks another? In Iraq, America is in its fifth year of learning the answer.

Petraeus' metrics of success might ignite more arguments than they settle. In America, police drug sweeps often produce metrics of success but dealers soon relocate their operations. If Iraqi security forces have become substantially more competent, some Americans will say U.S. forces can depart; if those security forces have not yet substantially improved, the same people will say U.S. forces must depart. Furthermore, will the security forces' competence ultimately serve the Iraqi state, or a sect?

Petraeus' report will be received in the context of his minimalist definition of the U.S. mission: "Buying time for Iraqis to reconcile." The reconciling, such as it is, will recommence when Iraq's parliament returns from its month-long vacation, come September.