After all the debate, secret briefings and public evasions of the past week, the nature of the European Union's defence plans remain as baffling as ever.
In Naples last weekend, Britain signed up to a deal with France and Germany which accepted but watered down their original proposals for an EU defence plan.
Mr Rumsfeld is unsure about "what is going to evolve"
Back in April, when transatlantic tensions over the war in Iraq were at their peak, France and Germany, together with Belgium and Luxembourg, proposed that the EU should have its own military planning headquarters, situated in the village of Tervuren, just outside Brussels.
The Naples deal had several new elements.
The EU's constitution, which it is hoped will be finalised by the end of the year, would include a mutual defence clause.
But at Britain's insistence, the text would state clearly that "commitments in this area shall be consistent with commitments under Nato, which, for those states which are members of it, remains the foundation of their collective defence".
Secondly, the military "headquarters" was scaled back to a mere "planning and operational capacity" - a "cell" situated safely within Shape, Nato's military headquarters near Mons in southern Belgium.
This is where current limited EU operations, in Macedonia and Congo, are operated from.
In addition, however, the EU's small strategic planning headquarters, situated in the heart of the "EU district" of Brussels, would be augmented and given an operational dimension.
Initially, only those countries "whose military capabilities fulfill higher criteria" would take part, under a system known as permanent structured co-operation, but others could join up later, subject to a majority vote of EU ministers.
So far, so good. The deal was seen as a triumph of British bridge-building.
The "hot-heads" of the EU (France's President Chirac had said in October, "There cannot be a Europe without its own defence system") had been reined in, and the Americans reassured that they were not being catapulted out of Europe.
Mutual defence clause
Yet the UK Prime Minister Tony Blair, on Tuesday, had this to say about the developments: "I am absolutely committed to European defence because I think it is important in circumstances where America is not engaged that Europe has the capability to act in these limited areas - because European defence is limited to peacekeeping, humanitarian and crisis management questions."
But if European defence is limited to those three purposes, then where does the Constitution's mutual defence clause fit in?
The draft wording of the clause states: "If a member state is the victim of armed aggression on its territory, the other member states shall give it aid and assistance by all the means in their power, military or other, in accordance with Article 51 of the United Nations Charter."
At Nato HQ, confusion reigns. As defence ministers met on Monday, both the alliance's Secretary General, Lord George Robertson, and the US Defence Secretary, Donald Rumsfeld, refused to say categorically that they were happy with the EU's latest plans.
Both appeared uncertain about how the proposals would develop.
Both simply refused to answer questions about why the EU needed a mutual defence clause and its own military planning structures, when both were already provided by Nato.
Open to interpretation
Lord Robertson claimed there was "still a mystery" about the EU documents.
Mr Rumsfeld said questions about the nature of the EU's planning cell were hypothetical: "We don't know what's going to evolve."
Both stressed that there was no problem about the new EU defence effort only inasmuch as it did not duplicate Nato or undermine its primacy in defending Europe.
At the moment, no European leader is publicly advocating such a major split from the defence arrangements of the past half-century.
But the Constitution and its annexes leave much open to interpretation.
There appears to be nothing, for example, to prevent the limited planning operation in Brussels from growing into something much more comprehensive.
That is what must worry the Americans. It is why, whatever text is finally agreed as part of the constitutional talks, the debate over European defence is likely to continue long afterwards.