The effort, partly financed by the United States and incorporating advanced American radar and other technology, has been progressing quietly for two decades. But Israeli defense and other analysts say it has now reached a level of maturity that could begin changing the nature of strategic decisions in the region. Centered on the Arrow 2 antimissile system, which has been deployed, the project is being extended to include a longer-range Arrow 3, the David's Sling interceptor designed to hit lower- and slower-flying cruise missiles, and the Iron Dome system intended to destroy Grads, Katyushas, Qassams and other shorter-range projectiles fired from the Gaza Strip and southern Lebanon.
With the Arrow system in operation and the Iron Dome due for deployment next year, Israel "has something to stabilize the situation: the knowledge that an attack will fail," said Uzi Rubin, a private defense consultant who ran Israel's missile shield program in the 1990s. Iran, he said, now cannot be assured of a successful first strike against Israel, while groups such as Hamas in Gaza and Hezbollah in Lebanon may find one of their favored tactics undermined.
Advances in Iran's rocket technology, coupled with its nuclear program, are chief concerns of the United States and Europe, as well as of Israel and other Middle Eastern countries. Alongside diplomatic efforts to persuade Iran to curb its nuclear research, missile defense programs have been designed with that country in mind.
The Obama administration decided this week to scrap a Bush-era plan to deploy a longer-range-missile defense system in the Czech Republic and Poland, and said it would move toward a more intermediate system that better matches its assessment of Iran's capabilities.
In Israel, the issue is considered a top foreign policy priority. There have been varying Israeli assessments about Iran's progress toward a nuclear weapon: The head of the Mossad intelligence agency told a parliament committee over the summer that Iran may be five years away from acquiring an atom bomb, but the head of military intelligence has said it could happen by the end of this year.
Prime Minister Binyamin Netanyahu, meanwhile, sees Iran's program as an imminent danger. It "is something that threatens Israel and threatens the region and threatens the peace of the world," he said during a recent visit to Germany. "There is not much time."
A recent unannounced trip by Netanyahu to Russia was thought by some Israeli analysts to be linked to the broad set of issues regarding Iran, including Russia's possible sale of advanced antiaircraft missiles to Tehran and the likelihood that Israel will strike Iran's nuclear facilities if the United States and Europe cannot find another solution.
But the steady growth of Israel's missile defenses sheds a different light on the country's military doctrine and sense of vulnerability.
Defense Minister Ehud Barak said this week that he did not consider Iran's nuclear program an "existential issue" because "Israel is strong." Part of that strength lies in its nuclear capabilities -- never acknowledged but widely presumed to exist -- and part in the assumption that the United States would stand behind Israel if it came under attack. But it also rests in the calculation that enough of the country's air bases and other military facilities would survive a first strike to retaliate effectively.
The sort of deterrence -- guaranteed retaliation -- that the United States and then-Soviet Union once achieved by deploying nuclear warheads in submarines and keeping bombers aloft is what Israel is striving for through its antimissile systems.
Iran "is radical, but radical does not mean irrational," Rubin, the defense consultant, said. "They want to change the world, not commit suicide."
Israel's program had its origins in the 1980s and grew out of concern about Syria's suspected acquisition of chemical weapons. It took on added urgency in the 1991 Persian Gulf War, when nearly 40 Iraqi Scud missiles hit the Tel Aviv area.
The Arrow was deployed in 2000, and Israel and the United States have since conducted a joint, biennial missile defense exercise, called Juniper Cobra, to work on integrating the weapons, radars and other systems of the two countries. Israel, for example, has the advanced U.S. X-Band radar stationed in the Negev desert. Israeli defense industry officials say the country also has almost real-time access to some U.S. satellite data, an important part of its early-warning system.
The next joint exercise is scheduled for October.
As concern shifted to the threat of long-range missiles from Iran -- the countries are about 700 miles apart at the closest point, well within the known range of Iranian missiles -- it also focused on the shorter-range weapons that Hezbollah and Hamas have turned on Israel in the past few years.
The rockets fired by Hezbollah at northern Israel during the 2006 Israel-Lebanon war led Israeli officials to accelerate work on a short-range-missile defense system, as did recent Grad strikes against Ashkelon, a Mediterranean city of about 120,000 people and the site of major electricity, desalinization and other facilities.
As it stands, "we have no defenses, no shelters, no public buildings being protected," said Alan Marcus, the city's director of strategic planning and architect of a plan developed to cope with the about 80 missile strikes since 2006.
"What do we do? Close the beach and tell people there might be a missile attack?" Marcus said.
Beginning next year, Israeli officials say, the Iron Dome system should provide some relief. The mobile launchers initially will be placed around towns and facilities near the Gaza Strip, but they ultimately may be deployed in locations nationwide.
The system has sparked some controversy. It has not, for example, proved effective against mortar shells and could leave the towns closest to the border areas vulnerable, including chief targets such as Sderot. Critics have pushed for other systems, including a chemical-laser one that Israel was jointly developing with the United States, or the rapid-fire Phalanx guns that can be used to protect key facilities such as power plants.
There is also concern that militant groups could try to overwhelm the system by firing large barrages of comparatively cheap, homemade Qassams -- perhaps not expecting to do damage so much as forcing Israel to spend tens of thousands of dollars a shot to knock them down.
But Israeli officials say systems such as Iron Dome are crucial to the country's military planning -- in terms of preventing damage and diminishing the need to retaliate.
Although many of the rockets fired by Hamas and Hezbollah land on empty land, "one of these times one of the Qassams will hit a bus, and then the government will have to make a decision" about how to react, said Shlomo Dror, a spokesman for the Israel Defense Forces. "There is a bigger issue here than how much it costs. It is going to give us some answers."