The problem of the formation of international security mechanisms inEuropein the late 90's of the twentieth century and the beginning of the twenty-first century is most closely connected with the transformation of NATO and the armed forces of the Allies. The urgency of research on this topic is conditioned by the need for reforming the Armed Forces of Ukraine in conditions of aggression of the Russian Federation (RF) againstUkraine, changes inUkraine's foreign policy course towards Euro-Atlantic integration and cooperation with NATO member-states. The purpose of the article is to analyze the stages, goals and directions of NATO's transformation in the context of transforming the geopolitical space of the European continent, through the prism of the last three NATO Strategic Concepts, after the end of the Cold War. The objective need to be able to withstand the present-day new threats has caused the need for changes in the functioning of theAlliance, which is reflected in the new and widespread notion of transformation. Transformation is a process that combines experience, innovative thinking, concept improvement, research and technology, experimentation, learning, and education to improve opportunities. NATO transformation determines the essence of the modernAlliance. The goal of transformation is the ability of theAllianceto meet the security situation and ensure that it effectively performs the tasks it has undertaken. In accordance with the decisions made, transformation covers a number of areas: political; military; organizational; structural educational and others. The three stages of the transformation of theAlliancecan be defined conditionally: the first – from the beginning of the 90-ies of the twentieth century until September 11, 2001; the second one – from 11.09.2001 before the beginning of the RF aggression againstUkraine; the third – from March 2014 and continues to the present. At the first stage, the imperative of transformation actually came to NATO since the end of the Cold War – the abolition of the Warsaw Pact (ATS), the collapse of theUSSR. NATO's historic decision to apply Article 5 of the Washington Treaty and to assist theUnited Statesafter the terrorist attacks of September 11, 2001, launched the second phase of NATO's transformation, characterized by the continuation of the transformational processes initiated in the first phase. The RF aggression againstUkrainerevived the risks of international clashes and showed that the territorial integrity of any country was not threatened and initiated the third stage of NATO's transformation, which continues to date and is characterized by the transition from the insurance strategy of the allies inEurope, to the containment strategy aggression of the RF. NATO's transformation is one of the most important conditions for achieving the Alliance's security objectives, its objective is to strengthen the Alliance's ability to counter existing and new security threats of the 21st century, to carry out a full range of military operations and missions on its territory and beyond. NATO transformation is a long and continuous process.