Aleppo is under siege and the prospect of the loss of thousands more civilian lives in Syria is very high. The UN has condemned the further descent to civil war but the fighting goes on with no sign of relief for Syrians. Jihadist elements have been drawn into the conflict. There is also high concern for the security of Syria’s chemical and biological weapons. The international community has seemed strikingly powerless in its attempts to influence the brutal course of events – but this is by no means inevitable.

While the Security Council is trapped in stalemate, so too is Syria. The government has attempted to suppress, through extreme violence, a popular and widespread movement that, after 40 years of dictatorship, has decided it can no longer be intimidated. The result has been an increasing loss of control on the ground, and the opposition has turned to its own military campaign to fight back. Yet, it remains unclear how the government can be brought down through force alone.

However there is also a political impasse. A mass movement, born in the demand for civil and political rights and the empowerment of voices for change, emerged in Syria after March 2011. But, for all the extraordinary courage that it took for the protesters to march each day in the face of escalating violence by the government, this did not become a movement that bridged Syria’s communal divisions. Opportunities to overcome this were then lost in increasing violence.

Military means alone will not end the crisis. Similarly, a political agenda that is neither inclusive nor comprehensive will fail. The distribution of force and the divisions in Syrian society are such that only a serious negotiated political transition can hope to end the repressive rule of the past and avoid a future descent into a vengeful sectarian war.

For a challenge as great as this, only a united international community can compel both sides to engage in a peaceful political transition. But a political process is difficult, if not impossible, while all sides – within and without Syria – see opportunity to advance their narrow agendas by military means. International division means support for proxy agendas and the fuelling of violent competition on the ground.

This is why I have consistently sought to help the international community to work together to end this destructive dynamic and to focus the minds of the parties on the ground into engaging in a political process. Early in my mandate we won international backing for this, with Security Council resolutions, which authorised UN military observers to deploy in Syria. After a ceasefire on April 12, contrary to some claims, the government’s shelling of civilian communities stopped, demonstrating the impact this unity could have.

Sustained international support did not follow, however. The ceasefire quickly unravelled and the government, realising there would be no consequences if it returned to an overt military campaign, reverted to using heavy weapons on towns. In response I sought to re-energise the drive for unity in June by creating the international Action Group for Syria, establishing a framework for a transition to support Syrians’ efforts to move to a transitional governing body with full executive powers. Transition means a managed but full change of government – a change in who leads Syria and how. We left the meeting believing a Security Council resolution endorsing the group’s decision was assured – as the first in a series of measures that would signal a turning point. But since then, there has been no follow-through. Instead, there has been finger-pointing and name-calling in the Security Council.

There are clear common interests among the regional and international powers in a managed political transition. A conflagration threatens an explosion in the region that could affect the rest of the world. But it takes leadership to compromise to overcome the destructive lure of national rivalries. Joint action requires bilateral and collective efforts by all countries with influence over the actors on the ground in Syria, to press upon the parties that a political solution is essential.

For Russia, China and Iran this means they must take concerted efforts to persuade Syria’s leadership to change course and embrace a political transition, realising the current government has lost all legitimacy. A first move by the government is vital, as its intransigence and refusal to implement the six-point peace plan has been the greatest obstacle to any peaceful political process, ensuring the distrust of the opposition in proposals for a negotiated transition.

For the US, UK, France, Turkey Saudi Arabia and Qatar this means pressing the opposition to embrace a fully inclusive political process – that will include communities and institutions currently associated with the government. This also means recognising that the future of Syria rises and falls on more than the fate of just one man.

It is clear that President Bashar al-Assad must leave office. The greater focus, however, must be on measures and structures to secure a peaceful long-term transition to avoid a chaotic collapse. This is the most serious issue. The international community must shoulder its share of responsibility.

None of this is possible, however, without genuine compromise on all sides. The stalemate means that everyone must shift: the government, opposition, international as well as regional powers. In this way, the international community can unlock an essential condition for a political process – a united international community, effectively and actively supportive of a peaceful transition to legitimate government.

Syria can still be saved from the worst calamity. But this requires courage and leadership, most of all from the permanent members of the Security Council, including from Presidents Putin and Obama. Is ours an international community that will act in defence of the most vulnerable of our world, and make the necessary sacrifices to help? The coming weeks in Syria will tell.

The writer, a former UN secretary-general and a Nobel Peace Laureate, resigned on Thursday as special envoy of the UN and League of Arab States for Syria