NATO bombed Serbia in order to make it a member of the Alliance. This goal, which for understandable reasons was not so obvious immediately after the air raids, became more and more obvious over time.
Further deployment of US troops in Poland will be a violation of the Founding Act signed between Russia and NATO in 1997.
On June 12th 2019, the United States and Poland signed a Joint Declaration on Defence Cooperation. The main agreements are to increase the presence of the US military and create special bases for them in Poland. Currently, about 4,500 US troops are based on a rotational basis in Poland. It is assumed that in the near future, the contingent will increase by another 1000 troops (relocated from the US military in Germany).
The Founding Act on Mutual Relations, Cooperation and Security between NATO and the Russian Federation signed in 1997 does not prohibit the deployment of NATO troops from other countries on the territory of new NATO members in Eastern and Central Europe. Thus, NATO treaties with Russia will not be violated, as the total number of military personnel in the European Union will not change, only a redeployment of troops will take place.