The latest U.S. general to take the helm in the Afghan war is facing major political challenges and tight timelines in his latest posting, says a retired Canadian major-general.

U.S. Gen. David Petraeus formally took command of U.S. and NATO forces on the weekend, days after U.S. President Barack Obama turfed his predecessor -- U.S. Gen. Stanley McChrystal -- after he gave a controversial interview to Rolling Stone magazine.

Retired Canadian Forces major-general Lewis MacKenzie says Petraeus is being thrust into a combat theatre where his allies are losing their resolve to continue the mission and his commander-in-chief is keen to see real progress in the near future.

Petraeus is also tasked with working with Hamid Karzai, the controversial Afghan leader whose shaky relationship with the U.S. government has been recently rattled by Obama's renewed intention to withdraw American troops from the war-torn country as soon as next year.

"The relationship is important, but it is fragile these days because of the unfortunate statement by the president of the United States that they would look at starting to thin out in 2011," MacKenzie told CTV's Canada AM during an interview on Monday morning.

Obama has said a troop pullout decision would be based on improved security.

MacKenzie said the word on the ground is that American military leaders will now "take a look at the situation in 2011 and see whether in fact they can start to thin some American forces out."

Combined with the fact that the Canadians are set to end their combat role in Afghanistan next year, while the Dutch and U.K. forces are looking at drawing down their own forces in the near future, Karzai is facing "a very tenuous situation" as he works out how he will handle the aftermath of the eventual NATO pullout, said MacKenzie.

That means that the Americans are left with little time to get their mission back on track, as Obama watches the results following his change of command in Afghanistan.

"President Obama has got some challenges to put together a team that can get along, have a common strategy and common objectives and start to see some success," said MacKenzie.

"Right now, we're at a very, very serious tipping point in that entire conflict in Afghanistan and neighbouring Pakistan."

CTV's South Asia Bureau Chief Janis Mackey Frayer said Obama needs to have something he can point to as a measure of success, or at least progress, as he heads into mid-term elections later this year.

"Public opinion polls in the United States are showing that people are turning against the war and certainly the Obama administration wants to be able to see something decisive in this campaign by the end of the year," she told CTV's Canada AM by telephone from Kabul on Monday morning.

Support for the Afghan war has also dropped in Canada and the United Kingdom, as the conflict drags into its ninth year and casualties continue to mount.

"There are some 130,000 troops who are going to be on the ground here by the end of the summer and the insurgency seems to be getting stronger," said Mackey Frayer.

MacKenzie said Petraeus will likely follow the counterinsurgency strategy employed by McChrystal, which the incoming general helped to shape when he helmed the U.S. war in Iraq.

"I don't think anybody down the chain of command will notice a change in strategy," said MacKenzie, noting that some small changes in tactics could be possible.

With files from The Associated Press