The United States and its allies have conducted over 5,000 airstrikes against the so-called Islamic State since August 2014.
Even though the air campaign has prevented ISIS from expanding its territorial gains on the ground, it has not deterred foreign fighters from joining the group in droves.
A recent report from the Homeland Security Committee estimates that there were at least 3,500 foreign fighters in Syria and northern Iraq in 2012.
In 2014, that grew to over 18,000 from 80 different countries.
By 2015, the number of foreign militants reached more than 25,000 from 100 different nations.
Tunisia is the largest origin country, according to the report, with no fewer than 5,000 fighters present in the region in 2015.
Saudi Arabia comes second with an estimated 2,275 while at least 2,000 more militants made their way from Jordan.
Russia recently joined the air campaign in Syria and it too is a significant origin country for militants.
The Homeland Security Committee estimates that there are at least 1,700 Russian fighters in Syria and northern Iraq while there are approximately 700 each from Germany and the United Kingdom.
by Niall McCarthy