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 al-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 assistance 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 C.I.A. said Thursday has as many as 31,500 fighters in Iraq and Syria, is already well-equipped and has proved to be stunningly skillful at waging war and seizing territory in both Iraq and Syria.

Despite efforts by the United States and others to persuade the insurgent groups to unify under a common political and military command structure, there is still no shared leadership. In fact, these groups may be close to defeat in Aleppo, where they are fighting both the Assad forces and ISIS.

In April 2013, Mr. Obama authorized the C.I.A. 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 C.I.A. began delivering light weapons like 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 up to $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 C.I.A. has been working for some time to vet the Syrian rebels, but on a limited scale; the expanded mission, which would include more fighters, is likely to make vetting even more difficult.

Beyond that, there are bigger questions. The main target of the United States right now is ISIS, but for the mainstream rebel groups, getting rid of Mr. Assad is the main goal. How do you reconcile those competing goals? How do you avoid a flare-up of anti-American sentiment? The Assad government and its allies Russia and Iran have condemned Mr. Obama’s plans, but how will they react when the military campaign begins? And how can weapons shipped to rebel fighters be kept out of the hands of ISIS?

America’s success at training security forces in other countries is mixed at best. Billions of dollars have been spent building up the Iraqi army, only to have key units collapse in the face of the ISIS invasion of Mosul. Unless the Obama administration can do better with the Syrian rebels, there is no chance the fight against ISIS can be successful.