Mr Aspin's tenure at the Pentagon has been marked by problems, related both to policy and to his own health. However, his decision yesterday to the leave the post was not expected and came as a shock even to close colleagues.
Mr Aspin said only that he was resigning for personal reasons. 'I have been working continually for 20 years to help build a strong United States military, so it is time for me to take a break and undertake a new kind of work.'
Although White House sources indicated that President Clinton is likely to announce his choice for a new Defense Secretary within days, Mr Aspin indicated that he would stay in his post until 20 January. This suggests that he will be attending the Nato summit with Mr Clinton in Brussels next month.
Speculation over the precise reason for the sudden departure had already begun in Washington last night. From the narrowest perspective, it is possible the Defense Secretary had lost patience over a budget wrangle with the White House involving additional defence cuts of dollars 50bn (pounds 33.5bn).
More broadly, however, Mr Aspin has been politically hurt by several perceived policy stumbles over the last months, notably arising from a decision not to honour a request from the military that he send reinforcements to Somalia just days before a clash in Mogadishu in early October in which 18 US troops died.
Mr Aspin, together with the Secretary of State, Warren Christopher, has come under attack more generally, especially from Congress and from political commentators, for what some consider a lacklustre and wobbly record on foreign policy so far, over Somalia, Bosnia and Haiti.
Mr Clinton last night did not stint in his praise for Mr Aspin, however. 'I will always appreciate the thoughtful and dedicated and selfless service that Les Aspin has given to this country,' he said. 'On a range of tough decisions and tough challenges abroad, from Bosnia to Korea, he has called them as he saw them.'
There is no hint yet as to who might get the call from the White House to take over from Mr Aspin. Although musical-chair sessions are not all that rare in US administrations, a void at the Pentagon is always awkward. Among the issues that remain urgent for the administration is the continuing concern over North Korea's intentions and the tension in that region.
Officials close to Mr Aspin insisted last night, meanwhile, that health concerns had nothing to do with his decision to leave. Early in the year the Defense Secretary was forced to withdraw for several weeks after having a heart pacemaker inserted.
Among his friends and colleagues, Lee Hamilton, chairman of the Senate Foreign Relations Committee, said the announcement had been made 'without warning'. He added, however, that 'I know he has been under great fire and personal distress.'
In military circles, there is likely to be private relief at the decision. Senior officers in the Pentagon were known to be unhappy with Mr Aspin's leadership, in part simply because of his donnish, baggy-suited style and his almost academic, thinking-out, way of policy- making.