Last week brought the good news that an unfortunate dispute between the United States and one of its best allies in Europe found the promise of a resolution. After meetings with officials in Washington, Polish Foreign Minister Radek Sikorski stated that the U.S. and Polish governments had reached an agreement in principle on plans to install a U.S. missile defense system on Polish territory, one dealing with Polish security concerns. It is an important step forward on an issue that had become dangerously stalled and a serious problem between allies whose close relationship predates the fall of the Berlin Wall.

Forward progress is critical to protecting Europe and the United States from long-range missile attack. Existing radar installations in the United Kingdom and Greenland are involved, as well as planned radar in Czech Republic and the interceptors planned for installation in Poland. It is clear that missile defense will increase levels of trans-Atlantic cooperation at a time when such relations are under strain from European Union ambitions for a separate defense identity.

Ronald Reagan embraced the idea of missile defense more than a quarter-century ago. As the United States slowly constructs its own land-based missile defenses, with interceptors first in Alaska and in California, the extension of missile defenses to Europe has had a rocky start. (It should be noted that sea-based missile defense cooperation with Japan has been speeded up greatly since North Korea fired test missiles over the Sea of Japan, which had a remarkably sobering effect on the opposition there.) Europeans still tend to fall into historic patterns of appeasing Russia and demand to know who they are being protected from. The answer is that today, the threat posed by missile attack is not from Russia, but a range of countries far more diverse than during Cold War days. Countries in the Middle East, notably Iran, as well as troublemakers like North Korea are arming themselves with medium- and long-range missiles. Twenty-five years ago, only nine states had ballistic missiles, today there are 27. Nine states have nuclear weapons, and many are aspiring.

Both the Czech and Polish governments have gone out on a limb, while other Europeans, particularly the Germans, continue to be nervous about any move that could upset the Russian government. In Poland, the fact that the previous government was unable to extract sufficient security guarantees from the U.S. government became part of the disillusionment with its performance, and the current government has been inclined to take a tougher line. In both negotiations Mr. Sikorski has been a key player, first as defense minister and now as foreign minister.

The Poles have worked to get a better bargain from the United States, and are reportedly considering waiting for the next U.S. administration to close the deal. This calculation, though, has surely been undermined by the fact that if the next occupant in the White House is a Democrat, missile defense as an issue will fall on hard times. The Democratic Congress has already cut funding for the development of the site in Europe. Were the negotiations between the Polish and the U.S. government to fall through, it could well have the consequence of bringing down the Czech government, which has stood by its end of the bargain.

The road to effective missile defense remains a long one, but the last week's development at least leaves that venue open. It has to be noted that Russian blustering about the system being directed at them is nothing more than that, as the trajectory of the unarmed interceptors makes it ineffective against Russian missiles. European critics have nonsensically even resorted to the argument that it could make a marginal difference toward making Russian missiles less effective overall, which makes one wonder what side they are actually on.

The fact remains this missile defense is purely defensive. Mr. Reagan had a way of hitting the nail on the head when it came to difficult policy questions. Once, when discussing with his arms-control experts the U.S. vulnerability to intercontinental missile attack, he put the problem this way, "Why don't we put on a helmet?"

It's a brilliant image. A helmet is at once purely protective - our soldiers wear them. So do our athletes, and our children wear them when they bicycle. A helmet is no threat to others whatsoever.

Helle Dale is director of the Douglas and Sarah Allison Center for Foreign Policy Studies at the Heritage Foundation.