1959 - Summits were held between Khrushchev and President Eisenhower in Geneva at Camp David, no agreement was made on Berlin but Khrushchev agreed to withdraw the Ultimatum
1960 - A U2 spy plane was shot down over the USSR just before its summit to Paris, Eisenhower refused to apologise for spying, so Khrushchev cancelled the talks
1961 - Khrushchev renewed the Berlin Ultimatum to new US president John F Kennedy and he did not want to appear weak and refused