WASHINGTON — If the temporary cease-fire in Syria begins to take hold on Saturday, despite the low expectations of the American and Russian officials who negotiated the agreement, it will be a landmark event. For the first time, diplomacy will have succeeded in abating the killing and misery that have already led to more than 250,000 deaths and millions of refugees pouring out of the shattered country.

But like everything else in the bloody five-year civil war, even the accord to limit the shooting has come at a high price, not least for President Obama.

In the estimate of European and Israeli intelligence officials, but not the White House, the pause in fighting may have the unintended consequence of consolidating President Bashar al-Assad’s hold on power over Syria for at least the next few years. Perhaps more important, if it proves successful, it may also begin to freeze in place what already amounts to an informal partition of the country, even though the stated objective of the West is to keep the country whole.

The “cessation of hostilities,” as it is officially known, that began midnight Friday is the first real product of a diplomatic push that Secretary of State John Kerry has made his all-consuming mission since last summer, when he struck the nuclear accord with Iran. Testifying in Congress this week, Mr. Kerry acknowledged that with so many players involved — Russian forces in the air, Iranian ground troops, dispirited and fractious opposition groups that say they have received too little aid from the West, and Mr. Assad’s own forces — this resembles a Hail Mary pass.