In recent years the UN Security Council has entered the scene of action several times after a unilateral military intervention has already taken place. The Security Council has adopted comprehensive schemes for the reconstruction of the countries intervened in and has authorised both civil and military international presences. Kosovo, Afghanistan and Iraq are examples of such recent situations, and Liberia is an example from the early 1990s. This article makes the argument that, through its resolutions, the Security Council contributes to the legalisation ex post facto of the unilateral interventions, whether it wants to or not. The Security Council is caught in a trap set by those who undertake the intervention without prior Security Council authorisation.The only way the Security Council could escape the retroactive legalising effect of its resolutions would be by clearly stating in the resolution its intention not to authorise the preceding intervention. Even then, it may be that the Security Council could not escape the power of its own practice. A persistent practice of adopting reconstruction resolutions ex post facto would carry greater legal weight than the professed intention not to legalise the preceding unilateral intervention.Still, authorisation ex post facto may be better than no authorisation at all.