An ISIS militant gestures at the Baiji oil refinery. With the QIWI payment system it's relatively easy for IS sympathizers to anonymously transfer funds to the extremist group with just a few swipes of their smartphone. Long War Journal The official propaganda of the Islamic State terror group declares that it wants to establish a "caliphate" that unites the world's Mulsims behind its strict interpretation of Sharia law, partly so that Muslims can be freed from the "persecution" they face in secular countries.

But there are other motives behind the group's grabs for territory.

The Islamic State (also known as ISIS, ISIL, and Daesh) has been seizing territory in Syria and Iraq for the past year ever since it declared the establishment of its "caliphate," an Islamic empire that aims to unite Muslims under a single religious and political entity.

While ISIS does aspire to global expansion, the group has so far been focusing on taking swaths of territory that are rich in oil, which it then sells in order to fund further expansion.

This month, ISIS has put up an intense fight in and around Baiji, Iraq, the site of nothern Iraq's biggest oil refinery complex. Both ISIS and the Shia militias that are fighting on behalf of the Iraqi government are throwing resources at the battle, including a reportedly heavy influx of foreign fighters, as they struggle for control of the key area.

As the below map of Syria from June 28th by Canada-based Syria conflict observer @KaryBDamoid demonstrates, ISIS has nabbed oil and gas fields as it advances across Iraq and Syria:

Twitter/@Karybdamoid

ISIS now controls most Syria's oil and gas fields. Extortion and taxation count for most of ISIS' funding, but oil revenues are still significant — The New York Times reports that in 2014, ISIS brought in $100 million from oil alone.

Airstrikes from the US-led anti-ISIS coalition are targeting the oil infrastructure ISIS controls in an effort to eliminate this funding source, the Times notes.

Expert Aaron Zelin tweeted this map, apparently from ISIS precursor Al Qaeda in Iraq (ISI) in 2006, that hammers home the oil strategy:

Aside from making money, ISIS seeks controls over certain energy resources as way to cripple its opposition, including the regime of Syrian dictator Bashar Assad.

Earlier this month, ISIS blew up a gas pipeline near the T-4 military airport in Homs province, according to 9 News. The pipeline carried gas into suburbs of Damascus and Homs and generated electricity and heat.

ISIS also took over the central Syrian city of Palmyra recently, giving the group control of key gas fields that serve as the "hub between the extraction or transfer of virtually all of Syria's gas production and the processing and power plants further west that supply electricity and gas for domestic and industrial use to those parts of the country where most of the population lives," according to Yezid Sayigh, a senior associate at the Carnegie Middle East Center in Beirut.

By taking Palmyra, Sayigh noted, ISIS is "depriving the [Assad] regime of 45% of its gas and electricity resources, according to Syrian opposition estimates."

Michael B. Kelley and Elena Holodny contributed to this report.