Starting right now on ABC's "This week," breaking news, Isis releases another chilling execution video. This time, a british hostage. How will the UK respond? Plus, president Obama making his case to... See More
Starting right now on ABC's "This week," breaking news, Isis releases another chilling execution video. This time, a british hostage. How will the UK respond? Plus, president Obama making his case to destroy the jihadist army. What does it mean for our troops, our safety? Are we really at war? This morning, answers, plus breaking developments from our global team. Football furor. That shocking video provoking a conversation about the hidden epidemic, domestic violence across America. And hawkeye bound. Hillary's first trip to Iowa in six years. Jon Karl and our powerhouse roundtable are on the road in Iowa. From ABC news, "This week with George stephanopoulos" begins now. Good morning. I'm Martha Raddatz. We begin with breaking news. Another horrifying execution video released by Isis. The victim, a husband and father, british aid worker David Haines. British prime minister David Cameron speaking just over an hour ago calling Haines a hero and warning that the UK must confront what he calls the Isis menace. We're covering all angles of the story this morning, and let's start off in the region with ABC's Alex Marquardt. Good morning, Alex. Reporter: Good morning, Martha. The brutal murder of british aid worker David Haines has been called by Britain's prime minister "An act of pure evil." Isis says it's retribution for Britain joining the american-led coalition against the militant group, a growing alliance that the U.S. Has been working on intensely for most of the past week to confront the Isis threat. David Haines had only been in Syria for a few days helping deliver aid to a refugee camp when he was kidnapped early last year. This weekend Isis released a grisly video of the 44-year-old's beheading, a warning they said to the allies of America. This latest chilling tape comes as the American military assault against Isis grows. So far, 160 air strikes against targets in Iraq, a campaign the U.S. Now wants to expand with the support of not just western allies but regional friends, as well, like Saudi Arabia where secretary of state John Kerry told me that he and the president want the largest coalition possible. Because the bigger it is, the more effective it is, the faster we can eliminate the threat. Reporter: While some have eagerly signed up, others have been more reluctance. Egypt said on Saturday it supports the mission but didn't say whether it would contribute. Turkey has been the biggest gateway for foreign fighters into Syria but wants to stay behind the scenes, and as Isis faces an international assault, its ranks are growing. A new cia estimate says Isis now has between 20,000 and 32,000 fighters. That number is only growing by the day, and so far despite those U.S. Air strikes, Isis has managed to hold on to most of the territory it has taken in Iraq and Syria. Even once this international coalition is finalized, the Obama administration admits it will take years to defeat Isis, a war that will almost certainly be handed off to the next administration. Martha. Thanks, Alex. More now on this horrific new video and the fallout. ABC's chief investigative correspondent Brian Ross is in New York, and ABC's hamish Macdonald is in London tracking all the reaction there, and, hamish, does this make it more likely that the british will get involved in military strikes with the U.S.? Reporter: Well, Martha, they certainly haven't committed to that formally yet, but David Cameron, the british prime minister, is saying that they are ready to take whatever steps necessary as this strategy against Isis intensifies. He's also using really the strongest language possible referring to this video on the beheading as "Callous and brutal." He says that Isis are the embodiment of evil, that these people are not muslims, they are, in fact, monsters. Of course, the british government has been fully engaged with this from the beginning given that the man who appears in the videos does seem to have a british accent. He's thought to be one of many british jihadists fighting in northern Iraq and Syria, but now to a british victim, that means that the government here is fighting against Isis on the home front, the rise of fundamentalism in Britain but also internationally, perhaps a significant development here today given that some senior islamic clerics have declared a fatwa against any jihadists who want to go and fight with Isis. They say they're heretics. Martha. And, Brian, why do they continue to put these videos out? It would seem to galvanize the world against them. Reporter: Well, Martha, they can't attack the U.S. Or great Britain as they're composed right now. But sadly the hostages, the Americans and now the british citizen serve as sort of stand-ins brought to their knees as if they bring the U.S. And Britain to their knees and then execute them. These videos also have the incredible potential to recruit more fighters to come and join them, and people we've talked to this morning, military analysts, say the fact they did this out in the daylight in the open at a time when they knew U.S. Drones are flying overhead shows they believe they can continue to operate with impunity despite the pledges by the U.S. To go after them. Thanks, Brian, and, thanks, hamish.
This transcript has been automatically generated and may not be 100% accurate.