Political tensions: Competition for spheres of influence, disagreements over political systems, disputes about territories, human rights issues.
Military threat: Both sides perceived each other as threats to their national security, led to an ongoing arms race with high levels of mistrust and tense situations like the Cuban Missile Crisis.
Economic competition: Capitalism vs. centrally planned economy, trade restrictions and sanctions that created economic divisions on the global stage.
Espionage and proxy wars: Accusations of espionage activities that created paranoia and led to proxy wars in various regions to spread ideological influence.
All these tensions combined to lead to a deep rivalry between the Soviet Union and the Western bloc during the Cold War.