Hundreds of thousands of Gazans reportedly remain in Egyptian Sinai, days after Hamas detonated the wall separating Gaza from Egypt. Hamas had reportedly planned its destruction of the border over the past few months, including by weakening the barrier so it could be toppled quickly by a few explosive charges. We can assume that Hamas also prepared for something else: sending terrorists into Sinai so they could infiltrate across the porous border between Egypt and Israel. Confirming this suspicion regarding Hamas's plans, Egypt says it has arrested some 20 armed Palestinians from Gaza who were equipped to enter Israel and launch attacks. The question now is, what will Israel do about its wide-open 240-km. border with Egypt? For years, the IDF has offered the government plans to erect a proper border fence, like the one that has largely prevented terrorists from crossing into Israel directly from Gaza. In 2005, just before Israel withdrew unilaterally from Gaza, including the Philadelphi corridor between Gaza and Egypt, then deputy IDF chief of staff Moshe Kaplinsky drew up a plan to build a proper barrier along the international border. The plan was called Project Hourglass. Back then, Hourglass was estimated to cost $1.5 billion. That is a lot of money, but as has been seen with the security fence in Judea and Samaria, even a partial barrier can be a major asset in preventing the infiltration of terrorists and catching them if they do. Ariel Sharon, Shaul Mofaz and Ehud Olmert have all traveled to the border and promised to implement the Hourglass plan. None of them, however, made sure there was money in the IDF budget to do so. The failure to secure the border is a disaster waiting to happen. Already, the lack of a proper barrier has led to rampant smuggling across the border, exacerbating the high rate of property crimes against farmers in the Negev. Long before anyone dreamed that Hamas would be able to send as many terrorists as it wanted into the Sinai, the risks from a wide open border with Egypt were considered unacceptable. It is obviously too late now to build a border fence to prevent such infiltrations in the near term. It is to be expected, accordingly, that the IDF has greatly increased its presence on the border. But even the most robust military presence requires a serious barrier to help do the job. It is already being suggested that, as has been the case until now, our military and political leadership will not find the funds to act on this critical matter unless there is a major terrorist attack that causes a public outcry for a fence. The same goes for the recent decision to stop funding security guards on buses, and the failure to complete the security fence in Judea and Samaria. While it very true that it is impossible to ensure security solely through defensive measures, and that excessive emphasis on defense can be a dangerous signal of a willingness to live with terrorism rather defeat it, defenses do play an important role in creating deterrence. The Palestinian suicide bombing campaign was defeated mainly by the targeted killings of terrorists and their leaders, but also because the security fence made the IDF's job much easier. Time and again, former Shin Bet head Avi Dichter testified that the security fence was critical to the successes the security forces had in preventing terrorist attacks. The same goes for the fence around Gaza, a barrier that was long resisted, in the same way Project Hourglass has been repeatedly put off, but in retrospect is seen as one of the best defense investments the country has ever made. The border with Egypt is not a temporary one, nor is it in dispute. As the US is discovering regarding its border with Mexico, even peaceful borders often need to be fenced off in order for nations to exercise their sovereignty and determine who may enter. The decision to build a border fence, accordingly, is not about a particular crisis, but the fulfillment of a basic long-term need. This need existed before Gaza fell to Hamas, and it will exist even if the threat from Hamas is removed. It is an investment worth making now that will benefit the nation far into the future.