"In the face of stiff resistance from Shiite militias aligned with Iran that have stalled their march on Baghdad, the ISIS fighters who captured Mosul in June pushed north during the weekend. By Sunday afternoon, they were in control of two other towns after fierce battles with Kurdish security forces, known as the pesh merga, who have been thrust into combat to defend the border of their autonomous region in northern Iraq from encroachments by ISIS.
In a statement, ISIS boasted of conquering “more important areas which were controlled by the pesh merga and the secular militias.” With the new territory, which the group described as “the border triangle of Iraq, Syria and Turkey,” ISIS strengthened its hold on territory that traverses the frontiers of Iraq and Syria, giving it an even greater ability to move fighters and weapons between the front lines of the civil wars in both countries." NY Times
IMO IS is systematically "cleaning up" its strategic "rear' in Iraq and Syria before continuing "march on Baghdad." I don't think this pause in the campaign against the Iraqi government has anything to do with Shia militia resistance. When IS finishes this phase of its campaign plan it will move to the next phase.
In this instance the Kurds lost important locations and populations to IS. Like the Iraqi government after the IS capture of Beiji and Tikrit, the Kurdish Authority will have one opportunity to demonstrate its ability to recover lost ground from IS. If that fails then the Kurdish Authority will be at the mercy of future IS "attention," and its fate will probably depend on Turkish, Iranian or US intervention.
With its program of Salafi renewal of supposed early Islamic practices, IS treats heterogeneous Islamic groups like all the Shia factions, Yazidis, Druze, etc. even worse that it does Christians or Jews (if there are any to be found in the "caliphate." ) These non-Sunni groups are simply considered to be murtadoon (apostates) and they are killed. Christians are offered the choice of payment of jizya (the dhimmi tax on second class people), conversion or the sword. pl