Microsoft has said it wants to buy the US arm of TikTok, the Chinese viral short-video app that is in the crosshairs of regulators and the White House.

But a deal, which is likely to run to several billion dollars, faces myriad technical and political challenges and will have to satisfy Zhang Yiming, the app’s founder, his powerful investors both in China and the west, and the Trump administration.

Microsoft said it would aim to complete a deal by September 15, but in an internal letter to staff on Monday, Zhang said the talks were “preliminary” and the exact details had not been worked out. Here is what is left to be decided.