I would absolutely believe that some day, given the opportunity, they would set up their state again, and that God would choose them anew, so changeable are human affairs.—Spinoza, Theological Political Treatise, Chapter 3 (translated by E. Curley).
I understand Zionism as a legitimate response to the reality that we do not live in a just world. In particular, for present purposes there are two pressing features of this injustice: first, there is no free movement of people (immigration and emigration). Our closed (or at best partially open) borders generate many global injustices. But here I only focus on the fact that this lack of freedom of movement makes minority groups especially vulnerable to local injustices because they can become, in effect, trapped behind borders. (I am not denying that there are some decent liberal and democratic arguments in favor of limiting immigration, but that need not concern us here.) Second, there are ongoing pressures in liberal democracies that generate enduring threats to the survival and dignity of minority groups; some of these pressures are a consequence of democracy and the intolerance or prejudices of electoral majorities (say, against Roma, the dark skinned, etc.), some of these pressures are the consequence of enforced state neutrality and hostility to various religious/cultural practices deemed barbaric, unethical, etc. (e.g., circumcision, ritual slaughter, dress codes, etc.). The pressures consequent democracy can be greatly mitigated by the rule of law, constitutional rights, and education (etc.); while the local details differ greatly, even the most robust constitutional states have ongoing difficulties in ensuring that all citizens are genuinely treated equally and with dignity. The pressures consequent the embrace of liberal values and a secular, even moralized conception of the public sphere generate ongoing tragic conflicts within liberalism pitting, say, impartiality and pluralism against each other. The first feature is a contingent fact about our world and represents the partial failure of liberal democracy. The second feature is, in part, a consequence of contingent local injustices, but may also represent, as long as there are liberal states, a permanent feature of social reality in which group identities and liberal values clash.* These two pressing features generate the structure to which Zionism is a legitimate response, but they are not sufficient for it.
The two pressing features mentioned in the previous paragraph point to the shortcomings and internal tensions of liberal democracy today. But Zionism is rooted, in part, in historical injustice. Jews are a historically persecuted minority. The previous sentence is not a uniqueness claim. The list of historically persecuted minorities is depressingly long (and may well get depressingly longer): in 'Europe' alone we also find, amongst other historically persecuted minorities, Gypsies, Roma, Armenians, Kurds, Basques. Non-European Liberal democracies in North and South America and Australia (etc.) offer their own examples of historically indigenous peoples and descendants of slaves that are historically persecuted minorities. To be a historically persecuted minority is a contingent affair dependent on balances of local power, demographics, economics, religious/natural rivalries, and state institutions. Not all minorities are persecuted, even if they have genuine political disadvantages (this describes the status of Catholics in my native country, the Netherlands, through the nineteenth century). Sometimes historically persecuted minorities stop being persecuted (or a minority). We may be witnessing, for example, the relatively rapid emancipation of gays and lesbians in many liberal democracies. It's because Jews are a historically persecuted minority and because the global political order has the particular character of injustice (recall the two features of the first paragraph above) that Zionism is legitimate.+
By 'Zionism' I mean to refer to a state that has a law of return for Jews. In this sense the State of Israel is Zionist. The point of the argument of the previous two paragraphs is to defend the legitimacy of a Zionist Israel against its critics. So, my argument is neither rooted in or about principles of international law (which is rather permissive of states and too harsh on non-state actors) nor rooted in ideal, moral theory (which is rightly deeply suspicious of any existing state short of a just world government), nor relying on Nietzschean will-to-power. It is a defeasible argument for a non-perfect world. I do not think my approach is original, but I am unfamiliar with a contemporary articulation of it.
While my approach has a moral tenor, it defends Zionism as a pragmatic, political solution to political problems. It is also an argument that bumps up against other legitimate, political aspirations (of Jews and non-Jews alike). It's not a Jewish argument in so far as that I recognize that one can be Jewish – and even accept my diagnosis and argument -- without being Zionist, and that one can also be a Jewish anti-Zionist (for religious and/or secular, moral reasons). The argument is, however, sensitive to Jewish experience, which explains why many Jews qua Jews understand themselves, in part, as a historically persecuted minority.**
Now, my approach is not rooted in democracy (and its purported legitimacy). While I am often impressed by the character of Israeli democracy and sometimes dismayed, there are inevitable tensions between the idea of Zionism and (liberal) democracy. To note this is to reiterate, in effect, the second pressing feature (with its tension between group identity and the demands of impartial morality) I noted in the first paragraph above. So, there is a sense that the very fact of Zionism is both a reminder of the internal tensions within liberalism as well as a limitation from the point of view of the highest ideals of modern liberalism. But I do not want to exaggerate Zionism's inherent flaws; in our imperfect world, the demands of group identity and democracy are constantly being renegotiated. (For example, the country I work in, Belgium, continuously ties itself into constitutional knots over competing claims of community and democracy.) All (liberal) democracies are experimental works in progress. I should also admit that while I am a staunch defender of democracy, its very form generates problems for minorities everywhere. For, in many ways, cosmopolitan, law-governed multi-ethnic empires may be better environments for minorities than liberal democracy developed from nation-states or colonial enterprises.