The killing of Iran's General Qassem Soleimani in a United States airstrikemarks a major increase in tension between the two countries.Many fear the incident could lead to war and instability in the Middle East.Soleimani was head of Iran's Quds Force, which has military operations in several countries in the region.The Quds Force is recognized by the U.S. as an organization that provides support to terrorism.Some considered Soleimani the second most powerful person in Iran behind Supreme Leader Ayatollah Ali Khamenei.Soleimani was killed along with several others Thursday near the Baghdad airport in Iraq.U.S. military officials said President Donald Trump ordered the strike to protect Americans.In a statement, officials noted, "General Soleimani was actively developing plans to attackAmerican diplomats and service members in Iraq and throughout the region."In Iran, thousands of people marched in Tehran and other cities to protest the killings.Supreme Leader Khamenei declared three days of national mourning for the general.He said "severe revenge awaits the criminals" behind the strike.The U.S. Embassy in Baghdad urged American citizens on Friday to leave Iraq "immediately."It said the embassy is closed and "U.S. citizens should not approach the Embassy."The Embassy was attacked by militia groups supported by Iran and other protesters earlier this week.Iraqi Prime Minister Adel Abdul Mahdi condemned the killingsas a violation of the conditions of the U.S. military presence in Iraq.He said the act of aggression violated Iraq's sovereignty and would lead to war.Russia's foreign ministry said that Soleimani's killing "will lead to a rise in tension in the whole region."And China urged "all the relevant parties, especially the U.S.,to exercise restraint and prevent the escalation of the tensions."The United States has about 5,000 troops in Iraq to train Iraqi forces and to fight Islamic State militants.Another 55,000 are positioned across the Middle East.All of them could be possible targets for Iran.Over the last 20 years, Iran has supported a network of fighting forces and militiasin several countries including Iraq, Syria, Yemen and Lebanon.These forces could present a threat to the stability in the region.Tensions between the U.S. and Iran have increased since President Trump's decision in 2018to withdraw the U.S. from Iran's nuclear deal with world powers.Last year, Iran shot down a pilotless U.S. military spy airplane,seized several oil tankers and interfered with international ship traffic in the Strait of Hormuz.The U.S. also blames Iran for a September attack on a large Saudi Arabian oil processing center.On Tuesday, a large group of protesters, angry over U.S. airstrikes targeting an Iranian-supported militia,attacked the outer edge of the U.S. Embassy in Baghdad.The attack led Trump to order another 750 U.S. troops to the Middle East.The embassy attack followed U.S. airstrikes Sunday that killed 25 Kataeb Hezbollah fighters.The militia, with ties to Iran, operates in Iraq and Syria.The U.S. said the strikes were in reaction to the killing of an American contractor in a rocket attack last week.Just hours before the news of Soleimani's death on Thursday,U.S. Defense Secretary Mark Esper warned, "The game has changed."