At least in Poland, EU and Poland start cooperate in autumn of 1988, when People Republic of Poland permanent diplomatic relations with the European Community. After collapse of communist regime in 1989, Tadeusz Mazowiecki govermnent in mid-september 1989 sign a treaty with EC about establish free trade principles and political and economic cooperation.

In same time Germany, France, Italy and rest of EC members start funding PHARE program ( Poland and Hungary: Assistance for Restructuring their Economies). hare funds focus entirely on the pre-accession priorities highlighted in the Road Maps and the Accession Partnerships which establish the overall priorities the country must address to prepare for accession and the resources available to help them do so.

Later, in 1991 Poland sign European Agreement establishing an association between Poland and the European Communities and their Member States. The agreement regulated economic cooperation and established a political dialogue between Poland and the EC. Furthermore, a provision was made in it which said that the association is only a means to achieve the goal of full membership for Poland and other Central-East Europe in the European Communities

Worth to note, in same time president Bush administration and US Senate work toward establish NATO membership with former Warsaw Pact countries. In 1989 for the first time in NATO's history, parliamentarians from Poland were invited to the Parliamentary Assembly of NATO in Rome. A year later, Poland and NATO established diplomatic relations. After the liquidation of the Warsaw Pact (1991) and the collapse of the USSR, Poland was temporarily in a zone of reduced security. In this situation, it was recognized that the alliance is one of the indispensable pillars of European security and in 1992 the aspiration to obtain full NATO membership was officially formulated.

Until 1994 Russia officially oppose joining Poland and other post-communist countries to NATO, making whole process a bit complicated for aspiring members.

So both sides want cooperate even when Cold War was still a thing. After collapse of Warsaw Pact and Comecom, both sides fairly quickly start deep political and economical cooperation to expand EC and NATO toward former communist bloc to reduce risk of deep political instability on continent for western countries and for post-communist countries got international funds and investments into collapsing economies and protect own independence.