Announcer: Starting right now on ABC's "This week" -- this morning, U.S. Air strikes against Isis inside Syria are imminent as president Obama takes his case against the brutal terror group to world... See More
Announcer: Starting right now on ABC's "This week" -- this morning, U.S. Air strikes against Isis inside Syria are imminent as president Obama takes his case against the brutal terror group to world leaders. As Americans we do not give in to fear. Announcer: The threat growing but can the militants really be defeated without Americans on the front lines? New NFL bombshell. Embattled commissioner roger Goodell breaking his silence, but can he survive the latest allegations? What the ravens really knew about that shocking elevator incident. And midterm shake-up. Surprising new twists in key senate races. Is republican momentum stalling? Why the senate is still up for grabs. From ABC news, "This week with George stephanopoulos" begins now. And we begin with the war on Isis. President Obama closing in on a decision to launch air strikes inside Syria as congress green-lights his plan to train and arm Syrian rebels fighting them on the ground but even the president's top military advisers question whether we can get the job done without American military troops. ABC correspondent Jon Karl starts us off. Reporter: With an assist from French pilots who are now flying their own missions over Iraq, the u.s.-led air war on Isis expanded. More targets, more bombs and strikes in Syria only a matter of time. The president heads to the U.N. This week to make the case yet again for a global campaign to destroy Isis. He's already made the pitch 5 times over the past 11 days. Each time also making it clear what he won't do. The American forces that have been deployed do not and will not have a combat mission in Iraq or in Syria. Reporter: The president's top generals seem to disagree. If there are threats to the United States, then I, of course, would go back to the president and make a recommendation that may include the use of U.S. Military ground forces. Reporter: The white house insists there is no disagreement, that the president is open to having U.S. Troops embedded with Iraqi troops on the front lines. Just don't call them combat troops. These would be troops that would be serving on the ground, these would be troops wearing boots, I assume. They would be combat ready and they would be in the line of fire. How are you not considering these troops that are combat troops? They are not in a combat role. They are in an advise and assist role. Reporter: Congress approved the president's plan to train rebels to fight Isis on the ground in Syria. The rare bipartisan show of support. But, wait, the Pentagon said it will take three to five months to begin the training and that the first fighters won't be trained until the spring at the earliest. It will take about a year to train the first 5,000. That helps explain why officials say the campaign to defeat Isis will take years and years. For "This week," Jonathan Karl, ABC news, the white house. And the success of this long war depends so much on the strength, skill and commitment of the Syrian rebels the U.S. Will be training and arming, many of them doctors and lawyers. ABC's bob woodruff brings us a rare inside look at the forces fighting both Assad and Isis. Reporter: As the fight against Isis in Iraq grabbed attention, what's referred to as the moderate rebels in Syria have largely been the forgotten story until now. We must strengthen the opposition as the best counterweight to extremists like isil. Reporter: Three years ago the in a popular revolt, the opposition rose up against dictator bashar Al Assad. Now they're taking on the vicious terror group Isis too. This week the leader of the moderates, Haider Al bahra, will address the U.N. General assembly to ask for assistance for a group president Obama once described as -- Blacksmiths and dentists, these aren't professional fighters. This statement makes me very proud of my people. This situation remind me greatly with American revolution. Who made this revolution? Wasn't it farmers, doctors? We are the normalcy fighting for our freedom. Reporter: Now the U.S. Is beefing up efforts to help them with funding and military training, which has led to some concerns. And we're told that if there are more weapons that are brought into Syria from the united States, that a lot of these people could shift to different groups based on how much weaponry they have. No, I assure you all the aid will go into moderate national Syrian army, the free Syrian army, and we'll be very careful with it. How long do you think it will take if you get what you want from the United States for this peace to come, for this war to end? If it's done correctly, it could be done within two to three years at max. Two or three years. Yes. Are you asking the united States to put some troops on the ground? We are not asking anyone to fight our own war. It's not just bahra's group in the fight. There are religious rebel groups taking on both Assad and Isis too. Vice news spent two weeks filming with the islamic front for a new documentary "Ghosts of Monday. Normal life during the war. One man tells us what else is there for me to do? They bombed my house. They bombed my house. That's it. So I've come back to fight. We found people trying to change their country. These were determined Syrians. Reporter: A determination among all the opposition groups in the conflict where the U.S. Is now getting more deeply involved. For "This week," bob woodruff, ABC news, New York.
This transcript has been automatically generated and may not be 100% accurate.