Detente was the policy of relieving tensions between the United States and the Soviet Union during the presidency of Richard Nixon. Nixon tried to ease tensions between the US and Soviet Union for multiple reasons including:
1) Both countries would benefit- If trade increased and the possibility of nuclear warfare decreased, citizens of both countries would greatly benefit. They would no longer have to be in constant fear of nuclear warfare.
2) Nixon's reelection campaign- Nixon's mishandling of the Vietnam War resulted in him needing to have a foreign policy accomplishment. This detente policy would help him win reelection.
The America defeat in Vietnam and the Soviet defeat in Afghanistan represent huge military failures by both countries. Both countries tried to help a potential ally but miscalculated/misunderstood the military tactics of their opponents.