In late 2014, with the war in Syria approaching its fourth year and Islamic State making gains in the north of the country, the exodus of Syrians intensified. At the same time, others were fleeing violence and poverty in countries such as Iraq, Afghanistan, Eritrea, Somalia, Niger and Kosovo.

Vast numbers of Syrian refugees had been gathering in border-town camps in neighboring Turkey, Lebanon and Jordan since 2011. By 2015, with the camps full to bursting and residents often unable to find work or educate their children, more and more people decided to seek asylum further afield.

In 2015 an estimated 1.5 million people made their way on foot from Greece towards western Europe via the "Balkan route". The Schengen Agreement, which allows passport-free travel within much of the EU, was called into question as refugees headed towards the wealthier European nations.

Tens of thousands of refugees were also attempting the perilous journey across the Mediterranean on overcrowded boats. In April 2015, 800 people of various nationalities drowned when a boat traveling from Libya capsized off the Italian coast. This was to be just one of many similar tragedies - by the end of the year, nearly 4,000 refugees were reported to have died attempting the crossing.

Countries along the EU's external border struggled to cope with the sheer number of arrivals. Fences were erected in Hungary, Slovenia, Macedonia and Austria. Asylum laws were tightened and several Schengen area countries introduced temporary border controls.

Critics of German Chancellor Angela Merkel's "open-door" refugee policy claimed it had made the situation worse by encouraging more people to embark on the dangerous journey to Europe. By September 2016, Germany had also introduced temporary checks on its border with Austria.

In early 2016, the EU and Turkey signed an agreement under which refugees arriving in Greece could be sent back to Turkey. The deal has been criticized by human rights groups and came under new strain following a vote by the European Parliament in November to freeze talks on Turkey's potential accession to the EU.

With anti-immigration sentiment in Europe growing, governments are still struggling to reach a consensus on how to handle the continuing refugee crisis. Attempts to introduce quotas for the distribution of refugees among EU member states have largely failed. Conflicts in the Middle East and elsewhere show no signs coming to an end, and the death toll from refugee sea crossings is on the rise.

The "Islamic State" (IS) — also known as ISIL, ISIS and Daesh — is an al-Qaida splinter group with a militant Sunni Islamist ideology. It emerged in the aftermath of the US-led invasion of Iraq in 2003 and is led by Abu Bakr al-Baghdadi. Their goal is to create a worldwide "caliphate." It gained worldwide notoriety in 2014 after a blitzkrieg military campaign that resulted in the capture of Mosul.

IS is believed to be operational in more than a dozen countries across the world. It controls territories in Iraq and Syria. However, the group has lost much of the territory it controlled in Iraq and Syria at the height of its expansion in 2014.

The US leads an international coalition of more than 50 countries, including several Arab nations. Russia, Iran and its Lebanese Shiite ally Hezbollah, which all support the Syrian government, also fight IS. Regional forces such as the Kurdish peshmerga (above) and US-backed Syrian Kurdish fighters, fight IS on the ground. The Iraqi army and militia have pushed IS from large parts of the country.

One of IS' main sources of income has been oil and gas. At one point, it controlled an estimated one-third of Syria's oil production. However, US-led airstrikes deliberately targeted oil resources and the Syrian government as well as US-backed Syrian Kurdish fighters have retaken most oil wells. Other means of income include taxes, ransom, selling looted antiquities and extortion.

IS has claimed responsibility for numerous terrorist attacks across the globe. The militant group has targeted capitals across the EU, including Berlin, Brussels and Paris. IS leaders have encouraged so-called "lone wolf" attacks, whereby individuals who support IS carry out terrorist acts without the direct involvement of the group.

The group uses various tactics to expand its power. IS fighters have looted and destroyed historical artifacts in Syria and Iraq in an attempt at "cultural cleansing." The group has also enslaved thousands of women from religious minority groups, including Yazidis. IS also uses a sophisticated social network to distribute propaganda and recruit sympathizers.

IS has further exacerbated the ongoing Syrian conflict. Millions of Syrians and Iraqis have fled their homes, many traveling to Europe in pursuit of refuge. Although it has lost all of its strongholds, the militant group has left extraordinary destruction in its wake. Areas affected by the militant group's rule will likely take years to rebuild.