The US has postponed tests on key elements of its controversial missile system in the hope that talks with a newly pliable Russia may yet remove major treaty obstacles.
US officials are now optimistic that Russia can be persuaded to radically amend the Anti-Ballistic Missile (ABM) treaty in exchange for significant mutual cuts in offensive missile stockpiles.
Briefing journalists on Thursday, the Secretary of Defence, Mr Donald Rumsfeld, announced that the US had delayed two planned tests because to have gone ahead might be seen in some quarters as breaching the 1972 ABM treaty, demonstrating what two months ago would have been uncharacteristic concern for the sensitivity of its allies.
The tests involved ship-based Aegis radars, used currently as a regional tool, tracking a long-range missile, an interceptor and a space rocket launch.
Such use of "theatre" radar for long-range monitoring is forbidden by the treaty.
The US, he said, was determined to demonstrate its good faith in negotiations with Russia about the treaty which Washington wanted to tear up. Mr Rumsfeld, who clearly did not personally share doubts about the legality of tests, repeated his insistence that the treaty was now outdated.
The test delays showed it was now an obstacle to US legitimate aspirations to build a missile defence system, he said, but "we have said we will not violate the treaty while it remains in force".
President Bush described the treaty last week in Shanghai as "dangerous". In his talks there on the fringes of the APEC summit with the Russian president, Mr Vladimir Putin, officials say, considerable progress was made in persuading Russia at least to amend the treaty.
Mr Bush argued that the recent attacks on the US underlined its vulnerability and strengthened the case for missile defence.
"We have an understanding that we can reach agreement," Mr Putin said, and diplomats here say there is a strong expectation of significant progress when the two men meet on November 12th at Mr Bush's ranch in Texas for three days.
On Wednesday, Mr Bush met the joint chiefs to discuss the US's nuclear posture and to examine how deep he can make the cuts in the country's nuclear arsenal which Mr Putin wants as a quid pro quo for the renegotiation of the Anti-Ballistic Missile treaty.
Mr Putin wants cuts in offensive missiles that go beyond the 2,000 to 2,500 level agreed between President Clinton and President Yeltsin in 1997.
Mr Bush is reported to be close to agreement with the Pentagon on the issue although it is complicated by calculations about China's and Pakistan's intentions.
In the wake of September 11th, Mr Putin was the first international leader to offer the US his sympathy and support and has shifted Russian foreign policy qualitatively towards greater engagement with the West, toning down criticism of missile defence.
Earlier in the year he had called the ABM treaty the "cornerstone" of international nuclear stability, but even if he accepts its amendment, opposition elsewhere is unlikely to disappear.
