There is no single kind of country that is most commonly affected by terrorism. Instead, there are at least two ways that we can describe countries that tend to have a large number of terrorist attacks against them.
First, we can say that terrorist attacks tend to happen in countries where at least a minority of people is very unhappy with the government. In these countries, terrorism is typically directed at the terrorists’ own government and/or civilians who tend to support the government. One example of this kind of country is Pakistan. The Islamists in Pakistan do not feel that the government is sufficiently Islamic and so they carry out attacks such as the one at the school for family members of the military in Peshawar in December of 2014. Another example of this is Afghanistan, where Taliban members and supporters tend to target government officials and their supporters.
In this sort of country, it is also possible for attacks to be made on foreigners who support the government. This is particularly true in places with a large foreign presence. For example, many terrorist attacks in Afghanistan have been conducted against Americans and other foreigners.
Second, we can say that rich and powerful countries tend to be the target of more terrorist attacks than other countries. We can see, for example, that countries like the United States and France have been targeted more than less “important” countries of the world. Terrorists tend to blame the rich and powerful countries for conditions that they, the terrorists, do not like. They do not blame less powerful countries such as Canada or Belgium because they do not see these countries as the causes of the way things are in the world.
Thus, we can see that different types of countries can be affected by terrorist attacks. Some of these countries are countries with problems caused by domestic terrorists. Others are countries that are powerful in the world and therefore draw attacks from people who are unhappy with the status quo in the world.
With respect to the Islamic terrorists, it is a matter of history that Israel and Jewish people around the world are the most threatened by them. Iran has long been an enemy of Israel; more recently, Israel suffered great harm from HAMAS, a Palestinian Islamic terrorist group. And, when Islamic terrorists struck Paris they wanted to attack a Jewish school, but settled for a kosher deli. Even more recently, the averted Belgium attack, the Islamic terrorists were stopped; however, in their investigations, police learned that
Mehdi Nemmouche, a French-Algerian ISIS fighter who allegedly helped guard Western hostages in Syria before returning to Europe, allegedly shot and killed four people at a Jewish museum in Brussels in May.
And, Jewish schools in Belgium canceled class after the news.
Many believe that it was no coincidence that the Islamic terrorist group al-Qaeda who destroyed the twin towers of New York City [9/11/2001] attacked these buildings which had many Israeli and Jewish employees; furthermore, New York City has the second largest Jewish population in the world. It is a matter of historical record, also, that the Saudi prince offered $9 million to then President George W. Bush if his government would "rethink" its position regarding Israel.