Ali Mamouri has written an article on why there is no love lost between Hamas and Islamic State (of Iraq and al-Sham) (see Why Islamic State has no sympathy for Hamas?). Ali argues that there are some doctrinal reasons why IS does not support Hamas:
Salafists believe that jihad must be performed under legitimate leadership. This argument is advanced through the “banner and commander” concept, which holds that whoever undertakes jihad must follow a commander who fulfills the criteria of religious and political leadership and has raised the banner of jihad. Given that there is neither a legitimate leader nor a Salafist-approved declaration of jihad in Palestine, fighting there is forbidden.So, there are two reasons:
In addition, for Salafists, if non-Muslims control Islamic countries and apostates exist in the Islamic world, the Islamic world must be cleansed of them before all else. In short, the purification of Islamic society takes priority over combat against non-Islamic societies. On this basis, Salafists see conflict with an allegedly illegitimate Hamas government as a first step toward confrontation with Israel. Should the opportunity for military action present itself in the Palestinian territories, Salafists would fight Hamas and other factions deemed in need of “cleansing” from the land and engage Israel afterward.
This approach has its roots in Islamic history, which Salafists believe confirms the validity of their position. Relevant points of historical reference include the first caliphate of Abu Bakr, which gave priority to fighting apostates over expanding Islamic conquests, which occurred later, during the second caliphate, under Umar bin al-Khattab. Likewise, Saladin fought the Shiites and suppressed them before he engaged the crusaders in the Holy Land.
Salafists today see that their priority as fighting Shiites, “munafiqin” (dissemblers, or false Muslims) and apostates, whom they call the “close enemy.” During the current war in Gaza, a number of IS fighters have burned the Palestinian flag because they consider it a symbol of the decline of the Islamic world, which succumbed to national divisions through the creation of independent political states. In Salafist doctrine, the entire Islamic world must be united under a single state, an Islamic caliphate, which IS declared in late June.
- According to Islamic doctrine, jihad is only allowed under a legitimate leadership and Hamas is not the legitimate leadership because it fights for democracy and a Palestinian state (while IS fights for God and for an Islamic state)
- IS has to fight the apostate Muslims first, before fighting the non-Muslims. Purification comes before conquest. So, fighting Shias in Iraq and Syria comes before fighting Israel.
IS spokesman, Nuseiri, also argued that it is following a systematic approach and has a plan (See ISIS Spokesman Explains Why 'Islamic State' Not Supporting Hamas)
However, he pointed out that ISIS has been taking a systematic approach in its campaign, and outlined six specific stages it said needed to be fulfilled before taking on Israel.
Some of those "stages" - building a firm base for an Islamic state in Iraq, and using it as a springboard to wage war in Syria and Lebanon - have already been achieved. But he said a number of other criteria still needed to be fulfilled before challenging Israel directly.
Among them, Nuseiri said that the US - seen as Israel's greatest ally - needed to be weakened politically and economically via attacks on the American mainland, as well as US interests in Muslim countries. Additionally, the existing "Islamic State" needed to expand its borders to cover all of "Greater Syria" (which would include Iraq, Syria, Lebanon, Jordan and possibly Gaza); such a state, he said, would then be in a position for a direct confrontation with Israel.
IS statement can be taken as few practical considerations (which every successful movement has to take into account) in the expansion of IS Caliphate and this is how IS wants others to see it. However, these doctrinal and practical considerations can also be understood as a camouflage for nationalistic concerns.
Doctrinally, the claim of legitimacy of IS is weaker than that of Hamas. Neither IS leadership is more pious, nor it has support/baya of most of the pious Muslim leadership. Moreover, jihad needs legitimate leadership when Muslims are attacking. When Muslim lands, homes and persons are attacked (as in Gaza), all Muslims are allowed/required to defend themselves, with or without legitimate leadership.
The argument about 'purifying' and killing apostate Muslims first and non-Muslims later is also weak. When Muslim women and children are being killed in hundreds, it is preposterous (and doctrinally indefensible) to not stop this massacre and focus on killing apostates. The fact of the matter is that IS's main enemies are Shias in both Syria and Iraq so it might also be way to legitimize its own actions. Moreover, as Salafist groups are challenging Hamas in Gaza, IS might want Israel to destroy Hamas so that more radical and doctrinally closer Gazan Salafists may take over.