European security depends on the effective collaboration of the five major powers; it will be undermined by the extension of NATO, a policy driven by US domestic politics. The main threats to security are: the breakdown of political and economic stability; unintended nuclear proliferation and/or failure of the START process; Russia's evolving political and territorial aspirations. All three will remain marginal as long as Russia is constructively engaged with the West. NATO expansion threatens that engagement. It is seen by all strands of Russian opinion as violating the bargain struck in 1990 and will likely lead to the withdrawal of cooperation. Invitations to Poland, Hungary and the Czech Republic cannot be rescinded, but the consequences can be mitigated by refraining from integrating them into NATO's military structure, by ceasing to insist that NATO membership is open to all, and by perpetuating the de facto nuclear-weapons-free zone that presently exists in Central and Eastern Europe. Britain's stance could be pivotal.