The shocking news reports were all too familiar: attacks using improvised explosive devices and suicide bombers in a major European capital, leaving scores dead and wounded. Madrid 2004. London 2005. Paris 2015. And now, Brussels 2016 is added to the grim list of scarred cities. But while the similarities are clear, the attacks in Brussels likely illustrate and portend radical changes in the terrorism and security landscape. The first seismic shift comes from Brussels's almost certain link to the November Paris attacks. Although the connection is not yet confirmed, early evidence suggests that the attackers in Brussels were closely linked to if not an integral part of the network that planned and executed the Paris events. Should this turn out to be the case - unlike closely timed copycat attacks of the past, such as that in London in 2005 - Brussels would mark the first time since 9/11 that a terrorist cell in the West survived to launch more than a single attack. And this despite four months of Europe's most intensive counterterrorism operations of the past 15 years by France, Belgium and other partner nations. If confirmed, the significance of this network's ability to survive cannot be underestimated and is likely driven by three factors: volume, sophistication and communications. With respect to volume, the sheer number of potential terrorists, especially ones who have received training thanks to the proximity of the Syrian conflict, is simply overwhelming European security services. With this volume has also come a level of sophistication - in planning, in staying "below the radar" and in creation of effective improvised explosive devices - that would have allowed this cell to survive the disruption of network safe houses and leadership and still move forward successfully. Finally, we must examine closely the extent to which widely available and simple-to-use methods of secure electronic communications could have allowed this cell to hide its electronic tracks post-Paris. In the past, once attacks occurred in the West, the United States and our allies were masterful in quickly obtaining and exploiting every bit of digital evidence associated with a plot. As a result, in 15 years no cells were sufficiently clandestine or resilient to continue to plot after being revealed by an attack. Although we never completely eliminated the masterminds in Pakistan, Yemen or Somalia, we always discovered enough to eliminate the group that had established roots in the West. I fear what we shall find in the Paris-Brussels network is that the group's communications were effectively hidden or sufficiently ephemeral to prevent security services from fully mapping the network that lived - post-Paris - to fight another day. Brussels does not simply represent change in the terrorists, it also portends great change (or future failure) for European security services. Post-Paris, the European Union's open borders were on life support; post-Brussels, the Schengen Agreement that requires such reliance on European counterparts is all but dead. As terrorists move freely about, a borderless Europe is already challenging, but when combined with bordered European security and intelligence services, the result is tragically predictable. The European Parliament will not give in easily, but a thickening of European borders, the establishment of robust, multilateral information-sharing arrangements and new European intelligence cooperation will be required to reduce the likelihood of future tragedies. The United States has a critical role to play in addressing both of these sets of challenges. As the West's technology center and intelligence leader, we must continue to work closely with our European partners and strengthen key bilateral and multilateral relationships. Moreover, we must re-learn - and re-teach - the painful lessons of the past decade-plus: Effective counterterrorism must balance a forceful mix of offense, defense and engagement with both par