Europe watched as the attacks on 9/11 unfolded across the Atlantic. The events would also change the life of the Old Continent with thousands of people being injured and hundreds of deaths at the hands Islamic extremists over the years.
Europe has seen many more terrorist attacks on its soil since 9/11 than the United States. Why? Analysts say there are many reasons.
Fernando Reinares, director at the Elcano Royal Institute Madrid's program on Violent Radicalization and Global Terrorism, says that "what we have seen in Western Europe over the past decade is an unprecedented jihadist mobilisation."
He says that there is evidence of this not only in the recent bombings, vehicle rammings, and stabbings in Western Europe, but also in the thousands of European Muslims who felt the need to join terrorist groups in the recent wars in Syria, Iraq, and Iran.
Western Europe has had a difficult time integrating significant Muslim populations into the mainstream of society. Many Muslims feel marginalized and disenfranchised and harbor grievances about the countries in which they live.
Peter Neumann, a professor at King's College London Security Studies, says that jihadists often latch onto a feeling of alienation or frustration.
Neumann, who is the principal advisor on security policy to candidate Armin Laschet during the current German election campaign, said that "that's not the case in the United States." "American Muslims are less hostile to their country than European Muslims and are more integrated."
In recent years, as a result of increasing influence from the Islamic State group propaganda, promises, soldiers returning from Syria or Iraq feel inspired to attack their home countries in Europe, causing alarm among European governments.
2001 proved to be a pivotal year in the history of jihadist terror activities in Europe and the United States. Olivier Guitta is the managing director of GlobalStrat in London, an international security-and risk consultancy firm.
He says that al-Qaida began to hunt for easier targets after the United States increased its security following 9/11. It took an opportunistic approach in Europe and recruited networks of support from Muslim communities to stage stunning attacks.
This strategy led to some very sad milestones in Europe. 2004 saw Madrid train bombings that killed 193 and wounded more than 2000 people. The London bombings of 7/7, also known as London Bombings, took place a year later. They featured coordinated suicide bomb attacks against public transport systems. 52 people were killed and 700 more were injured.
The Islamic State group was the main threat later. It was responsible for numerous attacks that left 130 dead and hundreds more injured in Paris, France in 2015. This attack was France's most violent since World War II. In 2016, 32 people were killed and more than 300 others were injured by nail bombs that went off in Brussels. A truck later drove into crowds in Nice in France killing 86 and injuring 434.
Some critics blame violence on weak links in Europe's defense. The 27 members of the European Union have different intelligence capabilities.
Daniel Benjamin, who was formerly Secretary of State Hillary Clinton's senior counterterrorism advisor, is now President of the American Academy Berlin. He says it's difficult to avoid this problem in a network of wealth and small-sized countries.
He says that "inevitably" there will be stronger and weaker intelligence and law enforcement communities in Europe. This is especially true for countries with so many resources.
GlobalStrat's Guitta says that anti-terrorism cooperation between EU countries has significantly improved since the 2015 Paris terrorist attacks.
This could prove to be valuable in the future. Reinares of Spain's Elcano Royal Institute predicts that al Qaida and Islamic State, who are rivals for prominence, will "compete to stage large attacks on the West." He also said that Europe should be cautious because it is a more easily target than North America or Australia.
Reinares claims that the continent lies closer to jihadist bases. Reinares also says it is more permeable internally, either through the lack of border controls across 26 countries, or through the migration routes used by tens and thousands of people every year.