The U.S. conducted its first airstrikes in Syria, a major expansion of President Barack Obama's effort to "degrade and ultimately destroy" the terrorist Islamic State.

"U.S. military and partner nation forces are undertaking military action against ISIL terrorists in Syria using a mix of fighter, bomber and Tomahawk Land Attack Missiles," Rear Admiral John Kirby, the Pentagon press secretary, said tonight in an e-mailed statement.

The U.S. is seeking to reverse the advances of Islamic State, a Sunni extremist group that has seized a swath of territory across Iraq and Syria. The U.S. has conducted more than 190 airstrikes against Islamic State targets, all of them in Iraq until now. ISIL is an acronym for the group's former name.

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Obama said in a televised speech on Sept. 10 that he would "not hesitate to take action" against the group "in Syria as well as Iraq."

"The decision to conduct theses strikes was made earlier today by the U.S. Central Command commander under authorization granted him by the commander in chief," Kirby said tonight in his statement.

While Iraq's government has invited the U.S. and other nations to help it fight Islamic State, no such request has come from Syrian President Bashar al-Assad, whose ouster the U.S. seeks.

U.S. Secretary of State John Kerry told a U.S. Senate hearing on Sept. 17 that in helping to defend Iraq, "you have a right of hot pursuit, you have a right to be able to attack those people who are attacking you as a matter of self- defense."

Russia's Opposition

Russian Ambassador to the United Nations Vitaly Churkin, whose country backs Assad, told the UN Security Council on Sept. 19 that any attacks inside Syria without Assad's approval would be "considered illegal" under international law.

Some U.S. allies have also shown reluctance to extend the strikes into Iraq.

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While France has joined the U.S. in airstrikes in Iraq, President Francois Hollande ruled out attacking in Syria.

"We're very concerned with the aspects of international law," Hollande said last week at a press conference. "We've been called in by the Iraqis; we're not called on in Syria."

Army General Martin Dempsey, Chairman of the Joint Chiefs of Staff, told a Senate hearing last week that about two-thirds of Islamic State's personnel, which the Central Intelligence Agency estimates at roughly 20,000 to 31,500, are in Syria.

Dempsey told the panel that attacks on Syria "will not look like 'shock and awe'" airstrikes that opened the 2003 Iraq War because that isn't how Islamic State is organized, "but it will be persistent and sustainable."

'Targeted Actions'

"This plan includes targeted actions" against Islamic State positions, including "command and control, logistics capabilities and infrastructure," Defense Secretary Chuck Hagel said last week.

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"Only in Syria can ISIS be defeated," said Jean-Pierre Filiu, a former French diplomat who served in Syria and is now a professor of Middle East studies at Sciences Po, a university in Paris. ISIS is an acronym for a former name of Islamic State.

"The head of the snake is in Syria," Filiu said by video teleconference at a forum today sponsored by the Washington Institute for Near East Policy.

As the U.S. expands its air campaign over Syria, the Pentagon has at its disposal manned B-1B bombers and F-16, F-15E and F/A-18 fighters. It also has Predator drones capable of dropping laser and satellite-guided bombs, including one with a 13-pound warhead called the AGM-176 Griffin.

Tomahawk Missiles

U.S. flexibility in hitting Islamic State targets also is aided by the capabilities of the latest version of Raytheon Co.'s Tomahawk cruise missiles, which can be redirected in flight to new targets. The Navy has warships in the Persian Gulf capable of launching Tomahawks.

The latest Tomahawk's "key advantage" is "you fly it and it can receive changes in targeting, changes in direction," the Navy's chief of operations, Admiral Jonathan Greenert, told reporters last year. "It can go up and actually loiter."

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Airstrikes are just the beginning of what will be needed to defeat Islamic State in what promises to be a years-long mission that ultimately will require some trained ground troops, said Michael Eisenstadt, director of the Washington Institute's military and security studies program.

"On its own, it won't be enough to defeat ISIS," Eisenstadt said at the forum today.

While Obama is counting on Iraqi and moderate Syrian rebel ground troops, "Our battlefield partners in Iraq and Syria are not ready yet," Eisenstadt said.

Training a 5,000-man force of Syrian rebels could take more than six months, said James Jeffrey, a former U.S. ambassador to Iraq who also spoke at the forum.