There is already a casualty of Barack Obama's anticipated strike against Syria: repeated warnings about the dangers of intervention voiced by his most senior military adviser.

General Martin Dempsey, the chairman of the joint chiefs of staff and former top army officer, has highlighted the risks of US involvement in Syria's bloody civil war for over two years.

Dempsey, a multi-tour command veteran of the Iraq war, has never openly opposed a strike on Syria, something that would risk undermining civilian control of the military. But when asked for his views, in press conferences and testimony, Dempsey has tended to focus on the risks and costs of intervention.

In April, Dempsey said that the US military could force down Syria's warplanes and disrupt its air defenses, but not without significant peril to US pilots, all for a negligible impact on dictator Bashar al-Assad.

"It's not about: can we do it? It's: should we do it, and what are the opportunity costs?" Dempsey testified to the Senate armed services committee in March 2012.

Dempsey's nomination for a new term as chairman was even briefly delayed in the Senate last month after pro-war senators demanded fuller advice about Syria.

In response, Dempsey listed nearly every military option mooted, from limited strikes to full-blown US intervention, and found them fraught with risk and expense. He emphasized the difficulty of staying out of the Syrian civil war once Washington launches any military action.

"Once we take action, we should be prepared for what comes next," Dempsey wrote to the committee on 19 July. "Deeper involvement is hard to avoid."

Even the "limited stand-off strikes" of the sort the Obama administration is now considering would require "hundreds of aircraft, ships submarines and other enablers." The impact on Assad would be felt "over time" in the form of a "significant degradation of regime capabilities," but there is a risk that "the regime could withstand limited strikes by dispersing its assets."

Tom Donnelly, a military analyst at the conservative American Enterprise Institute, said that Dempsey would be in a difficult position if the Obama administration, as expected, strikes Syria.

"Dempsey has always gone an extra step or two too far in being zealous in promoting the administration line, and now the rug's getting pulled out from him a little bit," Donnelly said.

Air force colonel Ed Thomas, Dempsey's new chief spokesman, distinguished the chairman's comments on US intervention in the Syrian civil war from the current administration deliberations about punishing Assad for allegedly using chemical weapons.

"His responses to Congress, articulating military options and risks, were about using military force in Syria to tip the balance of power, to contribute to a regime change," Thomas said.

"The discussion on military force today is based on a fundamentally different outcome – deterring Syria or any others from violating international norms and attacking with chemical weapons."

Donnelly considered Dempsey to have advocated staying out of Syria militarily, something useful for an administration wary of committing to a new Middle Eastern war.

"I think he made a policy statement in the guise of a military judgment," Donnelly said.

Few believe that Dempsey's standing inside the administration will suffer now that Obama is publicly considering a military strike on Syria. Nor is it likely that Dempsey would resign in the event of a strike, an extreme rarity for top US military officers.

"General Dempsey had a reputation inside the army as an absolute straight shooter," said Bob Killebrew, a retired army colonel.

"I don't think in terms of his present job he has any devious political motivation at all. He probably stepped over the line of civilian policymakers and military executors – I thought he was just a little over the line on that. But on the other hand he is walking on a political minefield now that is more fraught than any chairman ever has."

Dempsey's reluctance to intervene in Syria is likely "the opinion of all the chiefs" of the armed services, Killebrew added, as the service chiefs are more attuned to the dangers and uncertainties of war than civilians often are.

Additionally, "the Iranians see the survival of Assad as a vital interest to them, and I'm sure that what Dempsey sees is the broader war – not just between Assad and his opposition, but the broader war for Iranian influence in the Middle East," Killebrew said.

"I rather suspect that's the concern about being drawn in that he has, aside from any chairman's natural predisposition to be cautious."

Thomas, Dempsey's spokesman, said the chairman simply provided his best professional advice about the available Syria military options.

"The chairman provides military options to our elected leaders based on desired outcomes. He articulates the risk to both the mission and to our force, balancing our global responsibilities," Thomas said.

"And as the principal military adviser, he contributes to discussions about the use of the military instrument of power."