Arab plans for a joint military force are unlikely to go anywhere in the short term. Prospective member states lack the capacity and have to prove they have the will.

Egyptian president Abdul Fatah Sisi said on Sunday the countries currently carrying out strikes against Iranian-backed rebels in Yemen had agreed to form a joint force.

The announcement came after a two-day Arab League summit in the Egyptian resort city of Sharm el-Sheikh also attended by Yemen’s embattled leader, Abd Rabbuh Mansur al-Hadi.

The decision was primarily aimed at fighting jihadists who have overrun parts of Iraq and Syria and secured a foothold in Libya, the Arab League chief said.

Egypt had pushed for a rapid response force to suppress violent Islamists in the Middle East.

The countries may have been galvanized by the possibility that their ally, America, will do a deal with Iran under which the Shia state would forego nuclear weapons in exchange for an unwritten American acquiescence in recent Iranian strategic gains in the Levant.

Egypt’s Sisi has only been lukewarmly supported by the United Stats since he deposed the country’s elected Islamist president, Mohamed Morsi, in a coup two years ago. The Saudis wish America would have done more to support the largely Sunni uprising against the regime of Syrian president Bashar al-Assad.

The Saudi-led strikes in Yemen, where Iran seemed on the verge of winning another ally in the Houthi rebels, could be seen as the Arabs taking matters into their own hands.

But that doesn’t mean a unified military force would be successful.

When the Gulf Cooperation Council announced plans for a joint military command among its six member states last year, Shashank Joshi, a senior research fellow at the Royal United Services Institute in London, argued in Foreign Affairs magazine that the move was less revolutionary than it sounded. Arab states have often gone to war together, he pointed out, while joint military schemes, like the Gulf states’ Peninsula Shield Force, have usually been ineffective. When it was deployed to Bahrain in 2011 to put down protests led by the island’s majority Shia community, Kuwait and Oman simply refused to commit troops.

One reason that such attempts at Arab unity tend to fail is that smaller states are afraid of being overshadowed by their larger neighbors. Fears of Saudi domination have also preventing Gulf Cooperation Council states from forming an economic union and frustrated their efforts to erect a joint missile defense system, despite American backing and a shared fear of a nuclear Iran.

Moreover, many Arab states are embroiled in disputes over regional policy. Qatar’s ideological and material support for the Muslim Brotherhood has infuriated Saudi Arabia and the United Arab Emirates. Oman maintains closer relations with Iran than other Arab countries do. Just as there is not going to be a European army because the countries involved would disagree about how and where to use it, the Arab states have too many conflicting interests and priorities to make a permanent military force work.

Coalitions of the willing have been more successful. Most recently, warplanes from Gulf states joined American-led strikes against the Islamic State in Iraq and Syria while Egypt and the United Arab Emirates attacked Islamists in Libya in an effort to prop up the internationally-recognized government in Tobruk.

Without Western intelligence and support, however, the operation in Libya has so far done little to tilt the civil war in the Arab allies’ favor. General Khalifa Haftar’s private army has had more success at driving back the Islamist-dominated opposition still controlling Tripoli.

Joshi was probably right when he cautioned that the Arab states would struggle to mount “complex, sustained or demanding missions, such as those that would require destroying large enemy air-defense networks.”

Even if heightened fears of regional jihadism and Iranian hegemony would raise Arabs’ willingness — currently lackluster — to form something of a “NATO” of their own, they would still lack the capacity.