What was Plan Dalet?

Answer 1

General Discussion
Plan Dalet is an initiative that the proto-Israeli militias (like the Haganah or Palmach) had among a number of other initiatives to defend the nascent state of Israel from a civil war with the Arab population and invasions from hostile neighbors. Plan Dalet is often cited by those who argue that Israel had a clear and unambiguous intent to ethnically cleanse the nascent state of Arabs destroy or cleanse all Arab villages, but this perspective is not supported by its general understanding or application.

Benny Morris, a historian who has argued both for and against Israeli policy has stated that, "Plan D called for securing the areas earmarked by the United Nations for Jewish statehood and several concentrations of Jewish population outside those areas (West Jerusalem and Western Galilee). The roads between the core Jewish areas and the border areas where the invading Arab armies were expected to attack were to be secured."

Refutation of the Ethnic Cleansing Argument
The argument that positions and towns granted by the United Nations to Israel and those areas outside of it with large Jewish populations were to be held at all costs against invading armies says nothing about how Arabs civilians should be treated in those areas. The general policy practiced by the proto-Israeli militias in engaging with Arab civilians was to allow those that submitted willingly to Israeli control to continue unharmed, but to fight with those that contested the Israeli military presence. Abu Ghosh and several Arab towns around Jerusalem were spared violence by Israeli militias precisely because of this peaceful submission to Israeli authority. Similarly, a large number of Arab civilian towns surrendered in the Galilee region and were not forced out or ethnically cleansed. In fact, the numbers of Arabs who did not leave the Galilee were so high that Arabs have maintained a consistent majority of the population in the central Galilee since 1948 in Israel (in some places over 75% of the population).

Conversely, all Jews in what would become the West Bank or Jordanian-Occupied Jerusalem were forcibly evicted due to ethnic cleansing and those areas became Judenrein from 1949 until the Israeli conquest of the West Bank in 1967. If you are looking for ethnic cleansing during the Jewish-Arab Engagement of 1947-1949, it is much clearer in both word and deed coming from the Arab side against Jews than the Zionist Jewish side against Arabs.

Answer 2

A controversial set of guidelines to set up the state of Israel in 1948.