Was there a stated reason for the evacuation of Israeli settlers from the Gaza strip?

The Gaza disengagement plan was revealed to the public by Ariel Sharon in December 2003 at the Herzliya Conference, an Israeli policy conference for high-ranking politicians, security analysts and other bigwigs.^1,4 The plan was adopted by the Israeli cabinet in April 2004 and passed by the Israeli parliament in October the same year.^2,4 In Sharon's speech, he stated that:¹

The purpose of the Disengagement Plan is to reduce terror as much as possible, and grant Israeli citizens the maximum level of security. The process of disengagement will lead to an improvement in the quality of life, and will help strengthen the Israeli economy.

He further asserted that only settlements in territory that would not be included in the State of Israel would be evacuated and that Israel would strengthen its control over settlements in territory it intended to keep:

Settlements which will be relocated are those, which will not be included in the territory of the State of Israel in the framework of any possible future permanent agreement. At the same time, in the framework of the Disengagement Plan, Israel will strengthen its control over those same areas in the Land of Israel [Israel plus the West Bank and the Gaza strip] which will constitute an inseparable part of the State of Israel in any future agreement.

However, Sharon's plan faced stiff opposition both from within his own party Likud and outside of it. Benjamin Netanyahu, who would later become Israel's long-running prime minister, was one of its vocal critics. To appease the plan's many critics the scope of the disengagement and its rationale was changed.²

One argument added was that it would "serve to dispel the claims regarding Israel's responsibility for the Palestinians in the Gaza Strip".^2,3 Under international law the Gaza Strip is occupied by Israel and Israel is obliged to care for Gazans' well-being. But the Israeli government argued that after disengagement Gaza would no longer be occupied territory.

Another powerful agument in favor of disengagement was the "demographic threat". The idea that Israel is a "Jewish and democratic" state, but that too many non-Jews would force the state to choose between remaining Jewish or remaining democratic. If the state chooses democracy, Jewish dominance over the state would not be guaranteed and the state would become a secular multi-ethnic state. Perhaps like the U.S., where the white's dominance of the state apparatus is slowly eroding. But if the state chooses to be Jewish, it would need to implement apartheid against non-Jews to preserve its Jewishness and that would not be democratic.

The Israeli security think tank INSS in a policy brief from 2005 argues that the demographic threat was the impetus for the disengagement plan:⁵

For many years a large majority in Israel has understood the difficult and painful choices facing the country. One choice is to quit the territories and divide the region into two states that will leave Israel with narrower borders, but whose limited size is essential for ensuring that Israel remains a democratic state with a solid Jewish majority. The other choice is continued Israeli deployment in the Gaza Strip and the West Bank in order to retain control over all of the territory between the Mediterranean and the Jordan River, even if this results in the loss of a Jewish majority in the area within a short time and / or the end of Israel as a democratic state.

One person who understood the need for changing direction was Prime Minister Ariel Sharon, who initiated the current political plan that unilaterally cedes Israel's control of the land and the Arab population in the Gaza Strip. ... Sharon's assumption is that this step, which involves the evacuation of all Jewish settlements in the Gaza Strip (home to approximately 8,000 people), will free Israel from responsibility for 1.3 million Palestinians, an Arab population whose birthrate is one of the highest in the world.

A month before Sharon presented his plan at Herzliya his deputy Ehud Olmert warned about a need for radical policy change to avert the demographic threat in an interview with Haaretz:⁶

There is no doubt in my mind that very soon the government of Israel is going to have to address the demographic issue with the utmost seriousness and resolve. This issue above all others will dictate the solution that we must adopt. In the absence of a negotiated agreement ... we need to implement a unilateral alternative... More and more Palestinians are uninterested in a negotiated, two-state solution, because they want to change the essence of the conflict from an Algerian paradigm to a South African one. From a struggle against 'occupation,' in their parlance, to a struggle for one-man-one-vote. That is, of course, a much cleaner struggle, a much more popular struggle – and ultimately a much more powerful one. For us, it would mean the end of the Jewish state... the parameters of a unilateral solution are: To maximize the number of Jews; to minimize the number of Palestinians; not to withdraw to the 1967 border and not to divide Jerusalem... Twenty-three years ago, Moshe Dayan proposed unilateral autonomy. On the same wavelength, we may have to espouse unilateral separation... [it] would inevitably preclude a dialogue with the Palestinians for at least 25 years.

1. Address by PM Ariel Sharon at the Fourth Herzliya Conference-Dec 18- 2003
2. The Israeli Disengagement Plan as a Conflict Management Strategy, Yaacov Bar-Siman-Tov, Kobi Michael
3. The Cabinet Resolution Regarding the Disengagement Plan
4. The Politics and Economics of Israeli Disengagement, 1994-2006
5. "Two Roads Diverged": Israel's Post-Disengagement Strategic Options
6. 'Maximum Jews, Minimum Palestinians'

Theory vs. reality

In theory, these settlers could have remained there and lived under the new government of Gaza.

In theory this is correct. Moreover, in theory, once the Palestinian state is established, these people could become the (Jewish) citizens of this state.

In reality the tensions between the two sides are so strong, and recourses to violence are so common, that such co-existence would be very unlikely. Thus, the State of Israel was under an obligation to guarantee the safety of these settlers, because they are citizens of Israel and thus guaranteed protection by their state. This is further enforced by the Israel's declared mission to serve as a protector of Jewish people and victims of antisemitism.

On a lesser scale removing its citizens from danger zones or the areas of natural disaster, if necessary by force, is not an uncommon practice for many countries.

Practice
Removing of the settlers from Gaza is not a unique event. In fact, the settlement activity is restricted by the Israeli government. Firstly, because not all the parts of the West Bank are subject to the Israeli jurisdiction - following the Oslo Accords, the West Bank is divided into areas A, B, and C, depending on the degree to which they are administrated by the Palestinian government or Israel. Moreover, there are obvious restrictions related to the land ownership - the authorized settlements are build on the land owned by the state of Israel or the private Israeli citizens, but not on the land owned by the Palestinians or the Palestinian Authority.

This resulted in a phenomenon of outposts, which are the settlements built illegally from the Israel's point of view (I spell it fully, so that we do not get into a broader argument about the legality of the settlements). Dismantling and forceful evacuation of such outposts, ordered by Israeli courts is not uncommon, and extensively covered in Israeli news, see, e.g., here and here.

Remark: Many Israeli media also publish in English language version, notably Haaretz (major left-wing newspaper, roughly equivalent to Guardian/NYT/LeMonde/DieZeit) and Jerusalem Post (major right wing, roughly equivalent to .../WSJ/LeFigaro/FAZ).

Update
Another relevant case: evacuation of the Israeli settlements in Sinai peninsula, as a result of the Egypt-Israeli peace treaty in 1979.