The Strategic Arms Reductions Treaty (START) came to an end on Saturday, December 5—fifteen years after it entered into force in 1994. The treaty cut thousands of strategic nuclear warheads from the U.S. and Russian arsenals, introduced an impressive verification regime and provided a stable basis for the U.S.-Russian nuclear relationship.

In July, Presidents Barack Obama and Dmitri Medvedev laid down parameters for the START follow-on treaty. They agreed it would allow each side no more than 1,500-1,675 strategic warheads on no more than 500-1,100 ballistic missiles and bombers. Since July, the sides have narrowed the gaps. Expect the new treaty to set a warhead limit of about 1,600, plus or minus a few. The difference regarding the number of permitted strategic missiles and bombers now seems to be between 700 and 800.

As was the case with START, verification issues have emerged as tough problems in the end game of this negotiation. The Russians want an end to the U.S. inspection presence at Votkinsk, where they build their mobile intercontinental ballistic missiles. Mobiles pose a verification challenge. Their mobility makes them hard to target; that’s good for nuclear stability. Their mobility also makes them hard to count; that’s bad for arms control. Having inspectors at Votkinsk let the United States get a good count of the Russian mobiles. Without Votkinsk, the question will be whether there are alternative ways to get that count. This is probably a soluble problem.

A second verification issue is telemetry, the data that a missile and post-boost vehicle (the missile section that carries the warheads) broadcast during test shots. START essentially required the United States and Russia to share their telemetry, in order to make a missile’s performance transparent to the other. The Russians reportedly want to end this practice. This, however, appears more critical to Washington than Votkinsk.

So the sides have a bit of negotiating left to do. As Strobe Talbott writes in a companion Up Front piece, Moscow’s negotiating approach may have slowed the treaty’s conclusion. But Presidents Obama and Medvedev pledged on December 4 to work in the spirit of START and conclude a follow-on treaty “at the earliest possible date.” Both Washington and Moscow want a new strategic arms treaty. It’s a fair bet they will have one soon.