Prime Minister Theresa May took a gamble in announcing a starting date for negotiations over the terms of the U.K.’s divorce from the European Union.

True, there were strong demands from Brexit supporters at home not to delay departure. There are also good reasons not to prolong economic uncertainty longer than necessary.

But from a negotiating standpoint, by deciding to invoke Article 50 of the EU treaties and launch the two years of divorce negotiations, she takes away a big British bargaining chip.

Only London can trigger Article 50 and there is very little the other 27 members of the EU can do to accelerate it.

Officials in Brussels were expecting the U.K. to use that leverage over timing to encourage other governments to lay out their red lines in the divorce talks. As a result, they expected Mrs. May to attempt to bring the timing of Article 50 into the negotiations, and play on the impatience of the 27. So some were surprised when Mrs. May announced she would start negotiations by the end of March.

For the 27, starting the two-year clock running in March is helpful and may improve the atmosphere around the negotiations—not least because it means the British will be out by May 2019, when the next European Parliament elections are held. That simplifies legal issues over the status of British MEPs and other matters.

Holding Article 50 over the heads of her future negotiating partners like a Sword of Damocles would have soured any remaining goodwill among the 27, officials said. Many officials say that commodity is in short supply anyway, and attitudes to London haven’t been helped by some British cabinet ministers threatening to veto policies and predicting the post-Brexit doom of the rest of the EU.

The trouble for Mrs. May is that once the clock starts, Article 50 puts more power into the hands of the 27.

With everyone in the EU now accepting that Brexit will happen, some officials say all that is left is a mutual interest to have the divorce cause as little disruption as possible. There is little appetite to deliver what one called “goodies” to London.

Some say, for example, that financial passporting—by which banks in London can currently ply their trade throughout the EU—won’t be available unless the U.K. abides by the rest of the EU’s single-market rules, including those on free movement of people.

German Chancellor Angela Merkel signaled a tough line on Thursday. She warned German business leaders that the U.K. wouldn’t get membership of the EU single market—as many German manufacturers would like—if it didn’t also allow freedom of movement to EU citizens.

Given Mrs. May’s suggestion this week that controlling immigration was a priority over market access, Ms. Merkel seems to be preparing for a so-called Hard Brexit—a sharp break in relations.

European officials believe that once the negotiating clock is ticking, they are in the catbird seat.

Both sides want an agreement, they believe, but the U.K will need it more. After two years there is a “cliff edge”—with the risk for the U.K. that it will have to leave with no agreement, maximizing the disruption to its economy.

There is also an incentive for some of the 27 to play chicken, withholding agreement on important sections of the prospective accord until their demands are met. (The agreement will need only a qualified majority among the 27 to pass, as well as the backing of the European Parliament, but extending the two-year deadline does require unanimity.)

In all this, the U.K. does have leverage. It can provide contributions to the EU budget, easing the inevitable fight among the 27 that will ensue once the U.K. leaves, and it can make important input to foreign policy and counterterrorism. And EU companies want to keep trade flows with the U.K. unhindered.

But that is for the longer term. The most immediate question is whether the British can map out in advance their negotiating partners’ positions before the talks start in earnest. The public stance of the others is there won’t be talks until after Article 50.

Officials said the most Mrs. May is likely to get before March—and even then quite late in the day—is what one called a “landing area,” a broad-brush idea of what the future relationship will be like: the potential for a trade deal, cooperation in foreign policy and counterterrorism and perhaps some agreements in science and education.

There is no way, they say, that any single government can commit to anything before the 27 have set a common negotiating stance, and that won’t happen until after Article 50 is triggered.

The U.K. government has delivered the message to other governments that it wants no surprises in the negotiations. But its negotiating partners don’t see it the same way. From their standpoint, surprises will help them.