From roughly the time of the Crusades through the end of World War 1, Syria and the surrounding area was under the stable control of the Ottoman Empire. They were allied with Germany and Austria-Hungary during WWI, and so were on the losing side. The victorious French and British took over their lands in Levant (Syria, Lebanon, Palestine, modern-day Israel, and some other neighboring countries). After World War 2, the United States basically forced France and Britain to give up their colonies in the area for PR reasons. The French and British mapmakers who drew up the new territories did an atrocious job at this, randomly seperating tribes and ethnic groups, and forcing people who traditionally did not get along to live with each other (this also happened in Africa). Because the groups in these new countries wanted to have a bigger share of the political power, the United States and the Soviet Union (led by Russia) gave their support to rival groups. The United States, for example, helped install Sadam Hussein. After the end of the Cold War, the U.S. wanted to remove some of the "friends of the moment" from the Cold War who were actually pretty terrible human beings. For many reasons that don't entirely make sense, the United States decided to invade Iraq in 2003 to depose the dictator Sadam Hussein and install "democracy" in his stead. It was well intentioned, but there was very poor follow through, leading to political civil war and religiously motivated sectarian war among the country's Sunni and Shiite Muslims, as well as among the Kurds, a different ethnic group living in the north of the country that also has communities in Turkey and Syria. In 2011, there were many protests throughout the Arab world; people demanded more political freedoms and rights from the often corrupt governments. These protests also took place in Syria, which neighbors Iraq. When the Syrian government cracked down violently on the protests, it turned into a civil war. Because the government of Syria is a religious minority (Aluite, which is a sect of Shia Islam), Sunni Islam religious extremists associated with many of the insurgent groups in Iraq banded together. These groups eventually became more and more powerful, becoming the group that now calls itself the Islamic State, which invaded Iraq from its gains in Syria. The United States is now heading up a coalition of regional governments, along with some of its other military allies to try and stop the religious extremists from spreading. The United States would like to see the Syrian government removed, because they do bad things like murder civilians and have been implicated in using chemical weapons against civilians (made illegal by the Geneva Convention). Neighboring Arab countries want to remove the Syrian government for similar reasons, but also because most of them are Sunni Muslim, and the Syrian government is a sect related to Shia Islam. Iran plays a more complicated role. Their interests are aligned with and against the U.S. On the one hand, they are friendly with the Syrian government because Iran is primarily Shia, and so unlike the U.S., they do not want that government overthrown. On the other hand, they are also very friendly with the new Shia dominated government in Iraq (which the U.S. helped install after the invasion). So Iran is working with the United States in Iraq to stop the so-called Islamic State from taking over, but it is also working somewhat against the U.S. in Syria by supporting the regime. And Russia is currently sending weapons and soldiers to the Syrian government to help them survive because Russia sells a lot of weapons to Syria and in return, the Syrian government lets Russia have military outposts on their coastline. This gives Russia strategic naval bases in the Mediterranean. tl;dr: The British, French, U.S., and Russia (USSR) have various levels of culpability destabalizing the region, as do the often corrupt governments in the area. The Syrian government started a civil war by killing protesters. The civil war drew in religious extremists from Iraq who were veterans of the U.S.-led invasion, and are now attracting religious extremists from around the world. The religious extremists did so well in Syria, that they invaded Iraq. Now the U.S. has a coaltion of area governments fighting to stop them in Iraq and to a lesser extent Syria. Iran is helping the U.S. in Iraq, but supporting the government the U.S. doesn't like in Syria. Russia is now sending guns and soldiers to Syria, because the government has a history of buying guns from them and lets them have naval military bases on the Syrian coast.