Fifty-six years after the Elysée Treaty1 , the Treaty of Aachen signed on 22 January by France and Germany2 symbolises the will of both countries to continue to play the game of multilateralism and European integration. It also sets an objective of social and economic convergence and identifies sustainable development, environmental and climate protection as important areas of cooperation. The challenge now is for both countries to specify the course to follow and to implement the desired transformations in their national, bilateral and European policies.

The scope of this new Franco-German treaty is first and foremost symbolic. In a shattered geopolitical context, the message of the attachment and joint responsibility of France and Germany towards European integration, but also, more broadly, multilateralism and the United Nations Organisation, is welcome. The identification of the Sustainable Development Goals (SDO/Agenda 2030 for Sustainable Development) and the implementation of the Paris Climate Agreement and the protection of biodiversity and ecosystems as the foundations of the bilateral relationship between the two countries is a positive signal, as the response to these challenges requires the cooperation of all States. So how can the two countries jointly advance these objectives?

The first challenge is to agree on a common course. As such, the Sustainable Development Goals, an agenda for both solidarity and ecological transition, can serve as a compass. A Franco-German rapprochement would be beneficial on three levels: exchanging good practices to strengthen national implementation (French roadmap, German Nachhaltigkeitsstrategie), carrying this agenda, legitimate by its universal nature, as a new social contract at European level, and jointly preparing announcements of strong commitments for the first ODD Summit in September 2019 at the UN General Assembly.

With regard to climate change, it is of course regrettable that the objective of carbon neutrality, included in the Paris Agreement, has not been explicitly included in the Treaty. Nevertheless, it is its resumption within the European framework that will be decisive, while the countries of the Union are jointly committed within the framework of the Paris Agreement. The European Commission's publication of its climate strategy for 20503 , by making explicit the conditions for transformations to achieve carbon neutrality, has opened a debate that should have a key moment at the Sibiu summit scheduled to take place in Romania next May.

Secondly, this Treaty recognises the close partnership that exists in the development of European policies. It thus goes beyond declarations of intent and commits itself to "formulating common approaches and policies (...) by putting in place mechanisms for the transformation of their economies and promoting ambitious actions to combat climate change (...) through regular cross-sectoral exchanges between governments in key sectors". How can Germany and France jointly strengthen their ecological transitions? Two points seem important to highlight here.

On the one hand, the clear link between economic transformation and the fight against climate change can be welcomed in a treaty that aims more broadly at economic and social convergence between the two countries. Recent events in France, followed closely in Germany, point to the need to take environmental issues out of their political silos in order to succeed. Therefore, two questions must draw our attention to the implementation of the Treaty: will the ecological transition be taken into account in the elaboration of the common rules of the Franco-German economic zone and the the statements of the Franco-German Economic and Financial Council set up by the treaty? Will environmental issues be taken into account in the Forum's discussions on the Franco-German future? If so, this could strengthen the ability of the Franco-German couple to stimulate the ecological transition at the domestic level and at the European level.

On the other hand, the Treaty places discussions in a timely manner at the level of key sectors that need to be identified. It is the relevant scale to clarify the ins and outs of issues that are sometimes difficult to resolve between the two States. IDDRI has contributed and will continue to work on understanding these issues in the electricity system and soon in the mobility sector, with its German partners Agora Energiewende and Agora Verkehrswende. What is the balance between the rise in renewable electricity production, the exit of coal in Germany and the resizing of the nuclear fleet in France? What investments should be made in the European electricity network and what common tools should be implemented to support these transitions? What common vision on the future of the automotive industry and the signals to be sent to decarbonise car fleets? What cooperation between France and Germany, both producers of trains and aircraft, to invent and disseminate a long-distance mobility model that is consistent with a carbon-neutral objective? The exchange in the elaboration of national energy-climate plans that each European State must finalise before 2019, while the recommendations of the "Commission on the Future of Coal" in Germany4  have just been unveiled5  and the Multi-Year Energy Plan has just been detailed in France, is a first opportunity to formulate joint responses with their European partners.

Will Germany and France be able to give a new impetus to sustainable development with this treaty? The issues raised and sometimes difficult to resolve between the two States will certainly not be resolved by signing a friendship treaty. Nevertheless, it provides a framework for the bilateral relationship by recognising current practices and creating some new forums for dialogue. And it is through continuous dialogue that cooperation between the two countries can help to form virtuous compromises at the European Union level. It is now a matter for both countries to bring this treaty to life; they have all the tools in hand to do so.