The U.S. announced this week a major new air defense system for Israel, along with the deployment of 100 American troops to operate it in the country, in a development that will bolster Israeli security but potentially put American forces at risk.   President Biden said the deployment of the Terminal High Altitude Area Defense (THAAD) battery was meant “to defend Israel,” following Iran’s barrage of 180 ballistic missiles aimed at the country on Oct. 1. The U.S. ally is now weighing a retaliatory strike.   The Pentagon has described the deployment as part of larger adjustments the U.S. military has made in recent months “to support the defense of Israel and protect Americans from attacks by Iran and Iranian-aligned" groups in the region, according to press secretary Maj. Gen. Pat Ryder.  But a U.S. military deployment to Israel is unusual, with American troops generally aiding its ally’s defenses from Navy ships and fighter jets based outside the country — as it did when Iran attacked earlier this month and in April.   Defense experts believe the move could be an attempt to defuse tensions, as it might deter both Israel and Iran from larger retaliatory strikes.  “It might bind Israel from taking extreme steps, knowing that by doing so, they might [put] U.S. troops in harm's way,” said Eugene Finkel, a professor of international affairs at Johns Hopkins University.  The deployment also signals to the Iranians that “now they will have to deal with this extra layer of protection,” he added.  “This layer of protection is provided not by Israel, but the U.S. troops on the ground, and they might think twice about retaliation,” he continued.  THAAD, one of the most prized U.S. defense systems, can take out short, medium and intermediate-range ballistic missiles and is a critical part of the U.S. military's layered air defenses. It includes six truck-mounted launchers carrying eight interceptors each, a radar to detect incoming threats and usually takes 100 troops to operate.  It’s not clear when it will deploy in Israel, but its announcement comes as the country is considering a response to Tehran for its missile attack. The Washington Post reported Monday that Israel will not strike nuclear or oil sites and will instead aim for Iranian military targets, but that could still provoke a large response from Tehran. Israeli Prime Minister Benjamin Netanyahu was in a “more moderated place” in that discussion than he had previously been, the U.S. official told the Post, describing the call between the two leaders. The apparent softening of the prime minister’s stance factored into Biden’s decision to send a powerful missile defense system to Israel, both officials told the outlet. Israel’s counterattack could trigger an even “wider response” from Iran, with the U.S. likely sending the THAAD to ensure the protection of not just Israel but also American assets in the region, according to Ian Lesser, a distinguished fellow at the German Marshall Fund who heads the think tank’s program on southern Europe and the wider Mediterranean.  “It's an acknowledgement that there's tremendous uncertainty about what happens next,” he said. “If there was such confidence about this being contained, I don’t think we would see this.”  The full extent of U.S. forces in Israel is unclear. But the 100 troops deploying with the THAAD system will expand the risk of Americans in the country being targeted by Iran or Iranian-backed groups, such as Hezbollah in Lebanon.   Iranian Foreign Minister Seyed Abbas Araghchi warned as much on Sunday, saying that Washington was putting the lives of American troops “at risk by deploying them to operate U.S. missile systems in Israel.”  “While we have made tremendous efforts in recent days to contain an all-out war in our region, I say it clearly that we have no red lines in defending our people and interests,” Araghchi said on the social platform X.  Prior to Sunday’s announcement, the U.S. had sent Israel a THAAD battery after the Oct. 7, 2023, attacks and in 2019 for training, according to the Pentagon.  But the troops accompanying the system to Israel is a rare move for the U.S., which has stressed that it does not want to see a wider conflict emerge in the Middle East.  The Pentagon has been evasive as to how many U.S. troops are on the ground in Israel, where it operates a secretive facility in the country’s Negev desert called Site 512, used for radar and missile defense. Washington also opened its first permanent base in Israel in 2017 near the city of Beersheba, also in the Negev desert.  U.S. intelligence officers and special forces were also deployed to Israel after the Oct. 7, 2023, attacks by Hamas, who killed some 1,200 in the country and took 250 hostages, with Americans reportedly helping primarily with hostage recovery.  And over the spring, when the U.S. built and briefly operated a failed humanitarian pier off the coast of Gaza, American troops were working in the Israeli port city of Ashdod.  The U.S. already helped Israel defeat Iranian attacks in April and early October, using assets spread out across the Middle East. The U.S. has deployed an aircraft carrier strike group in the Middle East, as well as amphibious assault ships, a guided-missile submarine and numerous fighter jets.  Washington also has surged U.S. forces to the region, with as many as 43,000 troops in the Middle East after the Pentagon last month announced it was sending an “additional few thousand” individuals.  But the extra THAAD system will provide strengthened security for Israel’s air defense systems in the event of a large Iranian attack, and its deployment likely indicates a U.S. adjustment to Iran’s last attack, said Lesser from the German Marshall Fund.  “There may be systems the Iranians have not yet used that may be more capable and harder to intercept, and it may be that this system would provide some coverage against those,” he added. “There's something else that has been identified, a requirement that goes beyond what’s already been deployed, otherwise there would be no reason to do this.”  Sending another air defense system also indicates that the Biden administration is preparing for a potential worst-case scenario as the Middle East conflict spirals further out of control.  Israel has expanded its campaign into Lebanon to take on the Iranian-backed Hezbollah as it pushes to create a buffer zone and return some 60,000 displaced residents affected by more than a year of the Lebanese militant group’s rockets.   Israel also is continuing its war against Hamas in Gaza, where more than 42,000 Palestinians have died in more than a year of fighting. Around 100 hostages are still being held by Hamas.  Trita Parsi, the executive director of the Quincy Institute for Responsible Statecraft, criticized President Biden for sending the THAAD, arguing it enables Netanyahu to escalate the conflict against Iran and Iranian-backed groups.  “By defending Israel every time Netanyahu escalates the conflict in the region, Biden reduces the cost for Israel to escalate and to defy Biden's expressed wishes,” he wrote in a post on X. “If Biden wouldn't provide ADDED defensive capabilities to Israel AFTER it needlessly chose to escalate, the cost for escalation would have been higher to Israel — perhaps even prohibitive.”  Parsi added that Biden has “effectively put the US at war and needlessly gambled with the lives of American troops.”