The recent flurry of diplomatic activity aimed at reviving Israeli-Palestinian negotiations in order to avert a potentially destabilizing session of the UN General Assembly in September has focused attention on the territorial dimension of the conflict and, at the insistence of Prime Minister Benjamin Netanyahu, raised once again the question of “defensible borders.” Like many other phrases in the public debate, this one may sound commonsensical, but as George Gershwin once wrote: “It ain’t necessarily so.”In fact, unless it is set in the proper context, the notion of defensible borders comes perilously close to conceptual nonsense, and not just when it is deployed as a pretext to hold on to settlements.Geography and topography are undeniably important components of any country’s defensive capabilities. Throughout history, island nations like Britain and Japan benefited from maritime depth that made it much easier for them to defend against attempts by other to invade. Other natural obstacles, such as mountains or rivers, have also been rightfully seen as important defensive assets. Finally, territorial depth has conveyed critical advantage to defenders.That is why so many rulers have sought to move their borders further away from major national assets or, at least, to move those assets further away from the border (as Lenin did in 1918 when he moved Russia’s capital back to Moscow), and it is why Russia was able to absorb such horrific blows from Napoleon and Hitler but still ultimately prevail.These sorts of considerations provide part – but only part – of the explanation for the preference of many Israelis to make their borders as short and straight as possible, to exploit the few natural obstacles (waterlines and mountain ridges) that exist in the region, and to set the borders as far away as possible from critical national assets (the political and spiritual capital of Jerusalem, the demographical and economic hub of the coastal plain, and the transportation network, including Ben-Gurion Airport).Geography and topography are factors that exist independently of political or ideological bias. That is why recommendations for optimal border demarcation based on these factors resonate even among Israelis not moved by the historical/religious/emotional arguments for holding on to the Land of Israel. However, their objective character should not obscure the fact that they constitute only one part, and not necessarily the most important one, of the security equation.In fact, it is impossible to assess their significance seriously without also referring to the nature and sources of possible threats, the balance of forces, the state of technology, and, most importantly, the political framework. Other things being equal, it is better to have short, straight, naturally protected borders far away from critical national assets – but other things are never equal. In Israel’s case, territorial depth provides no defense against Iranian or Syrian missiles and is not needed against an eastern front that no longer exists. And borders fortified by mountain ridges or waterlines provide no protection against terrorism emanating from a hostile population within the borders.Finally, and most critically, there are the politics. Borders that best meet geographical and topographical goals will turn out to be a double-edged sword, a tactical asset but a strategic liability, because they will remain an obstacle to peace – not the only one, to be sure, and perhaps not even the main one, but an obstacle nonetheless. And retaining the tactical asset in the absence of peace will sustain the hostility of those on the other side of the border and impair Israel’s ability to bring to bear the other assets it has (military superiority) or to entrench those that it needs (morale, unity of purpose).In short, “defensible borders” as commonly understood by Israel’s governing elite are a mixed blessing. While they may provide some advantage in terms of geography and topography, they can also undermine the legitimacy of Israel’s use of force to defend itself and they can even, in extreme scenarios, corrode its viability as a national polity.Those who might consider this as hyper-sophistication flying in the face of common sense might do well to recall the response of Prime Minister Levi Eshkol, a man known for his commonsensical approach to complex matters. When asked in July 1967 about the wisdom of holding on to the West Bank after the Six Day War, Eshkol’s response was, “The bride is beautiful but the bride-price is too high.”They might also ponder the fact that Israel won its most brilliant military victory in 1967 fighting from what Abba Eban once described as “Auschwitz borders,” and that it paid a huge price to survive a close-run battle in 1973 fought from the most geographically favorable start-line one can imagine.Given the multitude of countervailing considerations that enter into any calculation of optimal security borders, it is not easy to formulate a persuasive reply to the question, “What are Israel’s minimum needs in terms of borders?” Even with the authority of people like Eshkol and David Ben-Gurion (whose approach was similar to Eshkol’s), it is difficult to calculate whether a longer border closer to the national airport will make people beyond the border – and even within it – hate you less and, if so, whether the tradeoff is worthwhile. But the debate about this would certainly be more productive if it were carried out with less passion and more reason.In any event, the search for an answer is not advanced one millimeter by simply declaring that the borders must be “defensible.”The writer is Principal Research Associate at the Institute for National Security Studies, Tel Aviv University.