Steinmeier is set to testify Thursday before a parliamentary inquiry into Germany's role in the Iraq war. He is expected to repeat the statements he has made several times in the past -- that Germany did not relay war-relevant information to the United States before and during the so-called Operation Iraqi Freedom.
Yet several U.S. military sources have stated pretty much the contrary.
Former two-star U.S. Gen. James Marks told German newsmagazine Der Spiegel that the intelligence collected and relayed to the U.S. military by two German spies stationed in Baghdad was "extremely important and valuable."
German intelligence was instrumental in planning attacks and even sped up the start of the initial invasion: It warned that Iraqi oil fields might be set on fire, prompting the U.S. military leadership to quickly press ahead with the invasion.
"We trusted the information coming out of Germany more than we did that from the CIA," Marks said.
Steinmeier's Social Democratic Party has since harshly criticized Marks' comments, accusing the former general of giving a skewed interview aimed at discrediting Germany's center-left half of the government.
But Der Spiegel bases its story on comments from roughly 20 military officials who all lauded the value of the German material, some 130 dispatched items, including positional data and photographs.
On Wednesday another high-ranking military figure backed the report -- Gen. Tommy Franks, the commander of the Iraq invasion.
"It would be a big mistake to underestimate the value of the information delivered by the Germans," Franks told Der Spiegel, which published his remarks on its Web site on Wednesday. "These guys were invaluable."
Steinmeier doesn't think so. He has said that the work done by the two agents from Germany's Federal Intelligence Service, or BND, was a humanitarian mission to relay non-military targets in order to prevent schools or hospitals from being hit.
That now seems like a bad joke. Of course, the intelligence provided by German spies didn't win the war, but it certainly influenced U.S. military decisions on where and when to attack, the U.S. sources told Der Spiegel.
"Whoever suggests that these reports played no role in the offensive is living on another planet," said Col. Carol Stewart, a member of Franks' staff.
This highly embarrasses Steinmeier, who has just been nominated to run against Merkel for German chancellor. Yet a legacy from his past may now lessen his chances.
At the time of the war, Steinmeier was a senior figure in Gerhard Schroeder's government, which had been re-elected because of its strong anti-war platform.
Merkel's conservatives have lost trust in Steinmeier, and of course they're using the affair to question his overall political credibility.
The Social Democrats have reacted by calling on the U.S. sources to testify before the committee.
Marks will be asked to fly to Germany, officials said. They can't force him to come. But according to Der Spiegel, he just may.