1. Involved in the murder of hundreds of Americans
On June 5, 1968, the presidential candidate Robert F. Kennedy was shot by the Palestinian Sirhan Sirhan shortly after midnight at the Ambassador Hotel, Los Angeles.
On 1 March 1973, in an attempt to free the assassin, Arafat and the PLO executed in Khartoum two American diplomats and one Belgian
On November 18,1979, shortly after U.S. diplomats were taken hostage in Iran, Arafat had ordered all Fatah cadres to help Iran’s revolution. U.S. intelligence discovered that this meant cooperation in terrorist operations.
When Arafat left Beirut in 1982, the many Lebanese who had worked directly for him were now given to the Iranians, who were increasingly promoting Islamist revolution in Lebanon, as their own network there. Perhaps the single most important member of this group was Imad Mughniyah. Born in a poor neighborhood in Beirut’s southern suburbs in 1962, Mughniyah had joined Arafat’s elite Force 17 security unit as a teenager and rose quickly from rank-and-file gunman to be an important Fatah intelligence figure. After joining forces with Iran, Mughniyah became head of the terrorist apparatus for Hizballah, the radical Lebanese Islamist group which took Americans hostage during the 1980s.
In 1983, Lebanese Shia terrorists bombed the U.S. embassy in Beirut and killed 63 Americans. Robert Baer, a CIA official who served in Lebanon and later spent years investigating that operation, determined:
“The only conclusion a reasonable person could make was that a Fatah cell — with or without Yasir Arafat’s knowledge — blew up the American embassy in Beirut on April 18, 1983, in cooperation with Iran and its agents.”
Arafat’s men may also have helped carry out the October 1983 attack on the U.S. marine barracks in Beirut, which killed 241 American soldiers. United States intelligence recorded a telephone call at the time in which Iran’s ambassador to Lebanon told Tehran how he had obtained bomb material from Fatah for a pro-Iran terrorist group to make the attack. 
2. Genocide of Lebanese Christians
On the 20th of January, 1976, Palestinian militias managed to besiege the town of Damour after they invaded the surrounded towns backed by leftist Lebanese militias.
Palestinian militias entered the Damour town and started massacring the Lebanese residents and forcibly displaced them. The militiamen closed all main entrances of Damour. Whole families were killed of whom the Kanaan family was the first, a family of a father, a pregnant mother and 4 children was brutally butchered.
“According to an eyewitness, the attack took place from the mountain behind the town. “It was an apocalypse,” said Father Mansour Labaky, a Christian Maronite priest who survived the massacre. “They were coming, thousands and thousands, Let us attack them for the Arabs, let us offer a holocaust”, and they were slaughtering everyone in their path, men, women and children.”
Nearly 600 Lebanese civilians were brutally killed and thousands more forcibly displaced out of their homes.
The American Lebanese League stated that the country had been “occupied by PLO terrorists” who “committed an orgy of atrocities and desecration against women and children, churches and gravesites… From 1975 through 1981 the toll among civilians was 100,000 killed, 250,000 wounded, countless thousands made homeless,” with 32,000 orphans and the capital city “held hostage by PLO criminals.”
Many years later, the World Lebanese Organization, the World Maronite Union, and multiple human rights groups concerned with the Middle East issued a public declaration accusing the PLO of genocide in Lebanon and addressing Yasser Arafat in these terms: “You are responsible for the killing of 100,000 Lebanese civilians… The United States government should have asked you to appear at the Hague for the crimes you perpetrated in Lebanon…” 
3. International legitimation through terrorism
In 2009, a former Italian president told reporters that his country had made an agreement with the Palestine Liberation Organization (PLO) and another terrorist group, the Popular Front for the Liberation of Palestine: the terrorists would limit attacks to Israeli and Jewish targets in exchange for public support for the Palestinian cause. On Friday, an Italian newspaper published reports supporting his account, and specifically showing that Rome declined to act on advance knowledge of the 1982 assault on a synagogue in which a two-year-old boy was killed and 34 people wounded.
From 1968–1984, 48 of the Palestinian terrorist attacks were carried out on German soil. The deadliest was the slaughter of the eleven Israeli athletes at the Munich Olympics in 1972. In response, West Germany recognized the PLO
In 1973, after the terrorist attack on the American embassy in Khartoum, The CIA reached an agreement with Arafat that the PLO would no longer attack American targets, with the US in return “recognize the legitimate rights of the Palestinians”
4. The establishment of Al Qaeda and ISIS
Abdullah Yusuf Azzam In Jordan, The man who eventually became the leader of the Afghan Arab mujahideen was a Palestinian national, Sheikh Abdullah al Azzam. He joined the Palestine Liberation Organization (PLO) in the 1960s
Abdullah Azzam laid the foundations of al-Qaeda, and from the war in Afghanistan to this day has been Osama Bin Laden’s spiritual mentor. In late November and early December 1989, on the basis of the idea of al-Qaidah al-Sulbah (“the solid base”)
Abu Musab al-Zarqawi, The founder of ISIS, was , the influential Palestinian Salafi jihadi ideologue Abu Muhammad al-Maqdisi
5. Inspiration for 9/11
In 1968, Arafat began his campaign of international terrorism, with backing from the Soviets. It was in 1968 that the PLO conducted its first airline hijackings. By 1980 PLO member organizations and front groups had carried out 42 acts of airline terrorism, including hijackings, bombings, ground assaults, and attempted surface-to-air missile attacks against jetliners.
Abu Ubeid al-Qurashi, an aide to Usama bin Ladin, who saw the PLO’s acts as a model for the September 11, 2001, attacks on the World Trade Center and the Pentagon. “the best proof” of terrorism’s value as a strategy was that Arafat was an honored guest at the UN General Assembly just eighteen months after his men gunned down athletes at the Olympic games. 
 Yasir Arafat: A Political Biography by Barry Rubin, Judith Colp Rubin pg. 97
 World Lebanese Organization et al., “Who is the Oppressor in the Middle East?” Washington Times, October 7, 1996, advertisement.
 “Chronology of Aviation Terrorism,” Skyjack: Aviation Terrorism Research
 Yasir Arafat: A Political Biography by Barry Rubin, Judith Colp Rubin pg. 73