Mr Hoon was succeeded as defence secretary by John Reid
A former defence secretary has said he opposed the deployment of British troops to southern Afghanistan in 2006.
Appearing before the Iraq inquiry, Geoff Hoon said the mission should not have gone ahead without a proportionate reduction of troop numbers in Iraq.
Mr Hoon, who left the Ministry of Defence in 2005, said the military did not believe it had the resources to mount both operations at the same time.
Opposition parties have accused ministers of neglecting troops' needs.
More than 9,000 British troops are now stationed in Helmand province in southern Afghanistan fighting the Taliban, the second largest contingent in the international coalition after the US.
More than 100 British troops were killed in Afghanistan last year.
The decision to send British troops to Helmand as part of a Nato campaign was taken after the 2005 general election, by which time Mr Hoon had moved to another ministerial role.
But he said his support for the Afghan campaign - while he was still at the MoD - had been "conditional" on the troop numbers in Iraq being reduced to prevent the army becoming over-stretched.
"What I was concerned about was the impact on our forces of trying to conduct those two operations simultaneously," he said.
"I felt it would have been better to draw down numbers in Iraq before committing ourselves to a Nato operation in southern Afghanistan."
He said he was influenced by the experience of trying to conduct two operations in both Bosnia and Kosovo, which he said "did lead to significant losses in our armed forces as people were simply being rotated from one theatre to another".
Asked whether senior military officers shared his concerns, he said they had wanted to be reassured that "they weren't going to be involved in two substantial operations simultaneously".
The UK withdrew the majority of its 4,000 remaining troops in Iraq in July 2009, having drawn down numbers over successive years.