Assertion: There was considerable unease in Indo-US relations during the 1950s.
Reason: India signed a 20-year Treaty of Peace and Friendship with the Soviet Union in August 1971.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

While India was trying to convince the other developing countries about the policy of non-alignment, Pakistan joined the US-led military alliances. The US was not happy about India’s independent initiatives and the policy of non-alignment. Therefore, there was considerable unease in Indo-US relations during the 1950s.