SILOPI, Turkey -- The United Nations’ decision to end its monitoring mission in Syria on Sunday robs the international community of an important window into the war-torn country and leaves the diplomatic road ahead uncertain.

The mission was unlikely to succeed from the beginning. U.N.-Arab League special envoy Kofi Annan’s six-point peace plan, which was supposed to have taken hold in April, required concessions neither side was willing to make.

However, the monitors did investigate some incidents and were able to assign blame. They also provided cover for international journalists to work in Syria. But the collapse of the mission appears to validate critics who deemed Annan’s last-ditch diplomacy “Mission Impossible.”

From the outset, Syrian President Bashar Assad’s regime never seemed serious about withdrawing its heavy weapons or releasing political prisoners, all parts of the agreed-upon plan. The opposition also refused to negotiate with the government as long as Assad was in power – a stance that the United States and other nations backed even though Annan’s plan called for both sides to name interlocutors and start talking.

The State Department doesn’t back an extension of the monitors’ mission, acknowledging that the warfare on the ground has outpaced efforts to end the bloodshed.

“We don’t think that they’re able to do the job that they were sent there to do, which was to monitor a cease-fire, which we don’t have, and to be able to move freely around the country, which they haven’t been able to do,” State Department spokeswoman Victoria Nuland said Thursday in Washington.

As the diplomatic impasse dragged on for months, hundreds more Syrians were killed in the 17-month-old conflict, which began as part of the Arab Spring protest movement and has swiftly evolved into civil war.

“All they do is come to count the bodies,” was a common refrain from rebels and anti-government activists in central Syria in June as the U.N. monitors documented grisly slayings that they said were carried out by the Syrian army and pro-government militias.

The end of the mission comes just days after a bombing in Damascus near the Dama Rose Hotel, where some of the remaining members of the estimated 300-member U.N. observer team were staying.

While rebels said Wednesday’s blast was meant to target a nearby Syrian military installation, it’s clear that many in the opposition view the observers as collaborators with the government rather than brokers or observers of a truce. But the monitors also assigned blame for the killings as the violence increased.

It was true that in areas under government control the monitors were flanked by Syrian military and intelligence personnel. This rankled many Syrians, especially when monitors passed through areas that were contested.

At times, even when Syrian government representatives were not present, residents of areas with open rebel sympathies often expressed the belief that the monitors were spies.

“I think it is unfortunate that we are losing the visibility that the observers provided,” said Daniel Serwer, a professor of international studies at Johns Hopkins University and a scholar at the Middle East Institute.

Referring to a mass killing in late May that left more than 100 mostly women and children dead, he said, “What they did in assigning responsibility for the Houla massacre was important. They are also the people who know both the opposition and the government officials best. Without the monitors, the international community is flying blind.”