Israel's military triumph over three enemy states in June 1967 is the most outstandingly successful war of all recorded history. The Six Day War was also deeply consequential for the Middle East, establishing the permanence of the Jewish state, dealing a deathblow to pan-Arab nationalism, and (ironically) worsening Israel's place in the world because of its occupation of the West Bank and Jerusalem.
Focusing on this last point: How did a spectacular battlefield victory translate into problems that still torment Israel today? Because it stuck Israelis in an unwanted role they cannot escape.
First, Israeli leftists and foreign do-gooders wrongly blame Israel's government for not making sufficient efforts to leave the West Bank, as though greater efforts could have found a true peace partner. In this, critics ignore rejectionism, the attitude of refusing to accept anything Zionist that has dominated Palestinian politics for the past century. Its founding figure, Amin al-Husseini, collaborated with Adolf Hitler and even had a key role in formulating the Final Solution; recent manifestations include the "anti-normalization" and the boycott, divestment, and sanction (BDS) movements. Rejectionism renders Israeli concessions useless, even counterproductive, because Palestinians respond to them with more hostility and violence.