In an interview on Friday, General Jones declined to speak about the memorandum. But he said: “On Iran, we are doing what we said we were going to do. The fact that we don’t announce publicly our entire strategy for the world to see doesn’t mean we don’t have a strategy that anticipates the full range of contingencies — we do.”

But in his memo, Mr. Gates wrote of a variety of concerns, including the absence of an effective strategy should Iran choose the course that many government and outside analysts consider likely: Iran could assemble all the major parts it needs for a nuclear weapon — fuel, designs and detonators — but stop just short of assembling a fully operational weapon.

In that case, Iran could remain a signatory of the Nuclear Nonproliferation Treaty while becoming what strategists call a “virtual” nuclear weapons state.

According to several officials, the memorandum also calls for new thinking about how the United States might contain Iran’s power if it decided to produce a weapon, and how to deal with the possibility that fuel or weapons could be obtained by one of the terrorist groups Iran has supported, which officials said they considered to be a less-likely possibility.

Mr. Gates has never mentioned the memo in public. His spokesman, Geoff Morrell, declined to comment on specifics in the document, but issued a statement on Saturday saying, “The secretary believes the president and his national security team have spent an extraordinary amount of time and effort considering and preparing for the full range of contingencies with respect to Iran.”