When Barack Obama chairs a special meeting of the UN security council on Wednesday it will be only the second time any US president has taken such a dramatic and symbolic step.

By bringing other world leaders to the famous circular arena, Obama seeks to dramatise the urgency of the issue at hand, the threat of the spread in the Middle East of a new extremist group, Islamic State (Isis). The US president is signalling his respect for UN institutions and an emphasis on forging a collective approach to the world’s problems. He is the only American leader to have convened such a meeting, and will now have done it twice. The first time was five years ago when the issue at hand was another global concern at the top of Obama’s agenda: nuclear proliferation.

The message is clear: he is nothing like his predecessor, George Bush, whose disdain for multilateralism and decision to lead the 2003 invasion of Iraq without a UN mandate, left deep rifts in the security council and in the wider world.

Yet, however hard Obama seeks to be the antithesis of Bush, the ghost of the Iraq war will hang over Wednesday’s proceedings. The president will arrive having just authorised the extension of the US-led air campaign against Isis in Syria, the legality of which is hotly contested.

The resolution that the security council will debate on Wednesday concerns cutting off international sources of financing and support for Isis and other terrorist groups – an aim that does in principle have concerted support inside the 15-nation body. But other states, led by Russia and China, can be expected to use the opportunity to pass judgment on America’s new war.

The resolution itself has some grey areas. It requires member states to stop terrorist groups such as Isis “recruiting, organising, transporting or equipping” on their territory or through their citizens. It will be framed under chapter seven of the UN charter, meaning it will be legally binding on member states, and a UN panel is supposed to assess compliance, but there are no enforcement measures envisaged. It will rely largely on states observing the spirit of the resolution. That will be challenging for US allies with a long and porous border with Syria, such as Turkey, and for Arab states such as Saudi Arabia where there is a lot of private funding for Isis.

The hope is that a binding UN resolution will make it easier for states to pass their own counter-terrorism legislation. It is less costly politically to be seen to comply with international law than to bow to US pressure. Yet for all of Obama’s multilateralist instincts, US leadership and foreign policy will be once more on trial at the security council at the start of a new Middle East war.