President Obama’s new strategy for routing ISIS, the extremist Sunni group that controls large areas of Iraq and Syria, rests substantially and precariously on having rebels in Syria fight ISIS, even as they battle the forces of the Syrian president, Bashar Assad. The plan is full of hope and fraught with obstacles.

During the three-year-long Syrian civil war, Mr. Obama has been rightly reluctant to provide significant weapons and military aid to the Syrian rebels. From the beginning, it was nearly impossible to determine the makeup and character of the rebel groups, of which there are about 1,500, according to James Clapper, Jr., the director of national intelligence.

Groups identified by Western intelligence agencies as the moderate opposition — those that might support democracy and respect human rights — have been weak, divided, and without coherent plans or sustained command structures capable of toppling the Assad regime. Today, those so-called moderates are even weaker and more divided; in some cases, their best fighters are hard-line Islamists.

In ruling out sending American combat troops into yet another Muslim country, Mr. Obama’s plan relies on these rebels to serve as ground forces to defend and seize territory after American airstrikes in Syria, for which he needs to seek congressional approval. But training and equipping them will be complicated and risky, and will take months, if not longer.

ISIS, which the Central Intelligence Agency says has as many as 31,500 fighters in Iraq and Syria, is well-equipped. It has proved stunningly skillful at waging war and seizing territory in both Iraq and Syria.

In April, 2013, Mr. Obama authorized the CIA to begin a secret mission to train Syrian rebels in Jordan. The total number trained so far is between 2,000 and 3,000. Last September, the CIA began delivering light weapons such as rifles and ammunition to a rebel faction commanded by Gen. Salim Idriss, whom Americans considered a competent leader and whose forces were not connected to terrorist groups.

But since then, the Supreme Military Council, which General Idriss headed, has broken apart, and he has been sidelined. Its weapons and supply storerooms have been looted by Islamist groups or stolen by its members.

As the ISIS threat became clearer, Mr. Obama announced a plan in June to spend as much as $500 million to send some American Special Forces troops to train as many as 3,000 rebels over the next year, but it stalled in Congress. Now the administration proposes training twice that number of fighters in neighboring countries in the Middle East, including a facility that Saudi Arabia has agreed to host.

One complication is the federal ban on sending military aid to people with a history of human rights abuses. The CIA has worked for some time to vet the Syrian rebels. The expanded mission, which would include more fighters, is likely to make vetting even more difficult.

America’s success at training security forces in other countries is mixed. Billions of dollars have been spent building up the Iraqi army, only to have key units collapse in the face of the ISIS invasion. Unless the Obama Administration does better with the Syrian rebels, the fight against ISIS cannot succeed.

First Published September 16, 2014, 4:00 a.m.