Prime Minister David Cameron has revealed in the UK but why do some people leave Britain to join Islamic State?
The extremist Islamist group, which is also known as IS, is mainly based in Syria and Iraq and is known for their cruelty.
They have been trying to get people from the UK to join them using social media and direct messaging online.
They target vulnerable young people who might be interested in what the Islamic State means. They promise adventure and fame, love, a perfect home, even their favourite chocolate.
But Islamic State doesn't tell the truth. It makes promises that it knows are not true.
This is called propaganda.
Instead of travelling to what has been described as an exciting place, the young people find themselves in the middle of a brutal war.
Girls are treated like slaves.
Conditions are bad, and Islamic State refuses to let people leave.
So, even if people change their minds, there is no way back.
Watch Ayshah's report to find out more.