Iraqi Mahdi army militiamen, loyal to the radical Shiite cleric Moqtada al-Sadr, carry their RPGs. Khaldoon Zubeir/Getty In a sign of the expanding international crisis sparked by ISIS's rapid advances, Syrian warplanes have bombed some of the jihadist group's bases in Iraq, Ali A. Nabhan and Matt Bradley of The Wall Street Journal report.

The bombing run was carried out in the western province of Anbar, which shares a long and mostly unpatrolled border with Syria. ISIS has found the space needed to plan and launch offensives in Iraq and Syria in the undefended and lawless frontier between the two countries.

This is the second consecutive day of Syrian bombings against ISIS targets in Iraq. At least fifty people were killed by the Syrian offensive, and another 132 people were wounded.

The Syrian bombing of ISIS marks a dramatic shift in strategy for the Assad regime. Until recently, Assad might have tolerated ISIS's existence within Syria — the group was a helpful bogeyman, and he could position himself as the lone alternative to a jihadist takeover of Syria. However, Assad began bombing the ISIS-held cities of Raqqa and Hasakah in Syria following the militant group's movement of heavy weapons, which had been looted from Iraq, into the cities on June 14.

Syria's bombing of ISIS positions in Iraq highlights the strange web of alliances developing internationally against the militant group. Iran has already sent 2,000 advance troops into Iraq to help protect key Shiite cities from the Sunni insurgency.

The U.S. is also sending 300 military advisors to Iraq to help check the advance of Sunni militants in the north and west of the country. However, President Obama has made clear that the U.S. will not send soldiers into combat in Iraq.