JUDY WOODRUFF: One year ago today, the Islamic State group shocked many around the world as it captured Mosul, Iraq's second largest city. One year later, Iraq is mired in a multifront war with the extremist organization and thousands have been killed.

The White House announced this afternoon that it will send more troops to train and advise Iraqis in the fight. Tonight, we will explore that decision and take a look back at the rise of the Islamic State in Iraq as part of our series No End in Sight.

JOSH EARNEST, White House Press Secretary: The president has been very clear that the efforts of the United States and our coalition partners will be to support the Iraqi people. We will not do for them what they must do for themselves.

JUDY WOODRUFF: The White House laid out plans to send up to 450 additional military trainers, joining 3,100 troops already there, and establish a fifth training base.

Spokesman Josh Earnest:

JOSH EARNEST: It reflects the need for the United States, our coalition partners and for the Iraqi government, to be nimble, as we respond to an adversary on the ground in Iraq that has also demonstrated a capacity to adapt their tactics and to try to capitalize on their perception of weaknesses.

JUDY WOODRUFF: All this follows the Islamic State's capture last month of Ramadi, capital of Iraq's largest province, Anbar. Government soldiers fled before the militants. And, Monday, at the G7 Summit, President Obama said there's no complete strategy for defeating the group until the Iraqis are ready.

That brought renewed criticism from Republicans, which House Speaker John Boehner echoed after today's announcement.

REP. JOHN BOEHNER, Speaker of the House: It's a step in the right direction, but as the president admitted the other day, he has no strategy to win.

JUDY WOODRUFF: Even today, White House and Pentagon officials said there's no major change in existing policy and no plans to send U.S. combat troops. But there will be at least one substantial effect: The new focus on Ramadi and Fallujah in Anbar province is likely to delay a planned campaign to recapture Mosul in the north, Iraq's second largest city.

It's been one year since Islamic State forces, in a lightning advance, seized Mosul, a strategic oil hub and a gateway to Syria. As in Ramadi, Iraqi soldiers put down their guns, and many shed their uniforms, leaving them in the streets for enemy fighters to find.

Triumphant militants paraded through the streets and refugees fled to camps in the Kurdish region, where thousands remain.

ABU ZEID AL-JABOURI, (through interpreter): Impossible. It's impossible to think that I have been out of the town for one year now, or that the current situation would take such a long time. I could have never imagined that on this date, I would be here in this place. It's a very sad and painful anniversary.

JUDY WOODRUFF: In the meantime, the Islamic State established a so-called caliphate across large swathes of Northern Syria and Iraq, and reached within 50 miles of Baghdad.

As they advanced, the militants carried out a reign of terror, including beheadings of hostages, brutal treatment of women, and destruction of ancient artifacts. And their influence spread on a global scale, attracting up to 31,000 new foreign fighters to the region.

President Obama had once played down the threat, likening the Sunni militant group to a junior varsity version of al-Qaida. After the fall of Mosul, he launched repeated airstrikes in Iraq and Syria. And he announced the initial effort to train Iraqi forces.

The fallout from Mosul also triggered a political shakeup in Iraq. Prime Minister Nouri al-Maliki resigned two months later, and Haider al-Abadi took over. He's since complained that U.S. help has been too little and too late.

HAIDER AL-ABADI, Prime Minister, Iraq (through interpreter): As you know, Islamic State wasn't born in Iraq. They are supported by means from outside Iraq, by external combatants. We can make sacrifices to fight, but the international community, the international coalition has to support us.

JUDY WOODRUFF: Instead, Abadi's government is relying heavily on Iranian-backed Shiite militias. They were critical to the recapture of Tikrit in April. ISIS also suffered a setback in Syria, where Kurdish Peshmerga forces helped drive the militants back from Kobani on the Turkish border.

But such gains have been greatly tempered by the losses of Ramadi and the ancient Syrian city of Palmyra. A big question will now be whether today's U.S. moves can help turn the tide.