NATO’s member states are willing to defend one another, and they have the troops and the equipment to do so. But quickly getting those troops and equipment to their destination is a different matter altogether. In some new NATO member states, bridges and railroads are simply not suitable for large troop movements. But one thing frustrates commanders even more: the arduous process of getting permission to move troops across borders.

“I was probably naïve,” admits Lieutenant General Ben Hodges, the commander of the U.S. Army in Europe. “I assumed that because these were NATO and EU countries we’d just be able to move troops. But ministries of defense are not responsible for borders.”

At their upcoming summit in Warsaw, NATO members will discuss joint responses to Russian aggression, and they are likely to agree to station four battalions—totaling about 4,000 troops—in the Baltic states and Poland. But with Russia forming two new divisions in its western military region, which borders the Baltic states, 4,000 forward-stationed troops may not be enough to deter a potential attack. (A division consists of 10,000 to 20,000 troops.)

U.S. troops drive during tactical road march Dragoon Ride II near Daugavpils, Latvia, June 6, 2016. Ints Kalnins / Reuters Indeed, “if NATO’s emerging forward enhanced presence is going to be more than a symbolic tripwire,” Ian Brzezinski, a deputy assistant secretary of defense under George W. Bush who is now a senior fellow at the Atlantic Council, told me, “NATO will have to conduct brigade- to division-level enforcement exercises in the Baltic region.”

And beyond deterrence, if a member state were attacked, NATO commanders would need to quickly pull together troops from different corners and bring them across Europe to their destination, which would most likely be a new NATO member state.

And there’s the complication. Moving troops across Europe requires permission at each border. “During the Cold War, we had pretty good plans to rapidly move across borders, but until [the 2014 NATO summit in] Wales we didn’t have similar plans for new NATO member states,” says a NATO official knowledgeable with the issue. “Right after Crimea we sent out a questionnaire about [border regulations] to each member states, and the results were pretty scary. Some countries needed to recall parliament in order to let NATO units cross their borders. And one country said, ‘we can only have 1,600 soldiers on our soil.’” In reality, that meant that NATO would be unable to use that member state, which the NATO official declined to identify, for passage.

A specialist inspects a U.S Air Force F-15 Eagle fighter after a certification of the arresting gear in the military air base in Lielvarde, Latvia, May 19, 2016. Ints Kalnins / Reuters Since then, NATO has made impressive progress. It has tripled the size of its 13-year-old NATO Response Force (NRF), which can muster up to 40,000 troops and is, at least in theory, able to deploy quickly to new NATO member states as well as old ones. And all of its member states have agreed to pre-clearance—the military version of a green card for troops and equipment—although it is not clear how the system will work in practice. As the NATO official reports, “some countries say ‘we don’t need any advance notice for pre-clearance,’ while others say they need four to five days’ notice.” According to the official, in most of NATO’s eastern-facing countries, getting the clearance would be a matter of five days or fewer, although one country—he declined to specify which one—still requires more time.

And so, although Hodges and his fellow commanders know how fast their troops can physically move, they have little idea whether crossing borders will take five days, two days, or perhaps just hours. “An official [in an eastern European NATO member state] told me, ‘I hope we can get this [clearance] done quickly,’” Hodges reports. “But you can't plan based on hopes and wishes.”

SACEUR, NATO’s Supreme Allied Command in Europe, does, however, have clearance for access to Polish and Baltic airspace. And according to a NATO spokesperson, almost all NATO member states now have legislation in place for the deployment of the alliance’s 5,000-man VJTF rapid response force, usually referred to as the Spearhead Force. The VJTF, established at the 2014 Wales Summit, is NATO’s rapid response to any attack on a member state’s territory. It operates on a rotational basis and does not have a permanent home. The reaction time of the quickest units of the VJTF will be measured in hours, and the first troops will deploy to the crisis area within 48 hours.

In late May, the Spearhead Force conducted a 2,500-man Brilliant Jump exercise, which focused on the logistics of moving from Spain to Poland within four days. It succeeded, but the question is what will happen when more new NATO members’ borders, and larger numbers of troops, are involved. Estonia, for one, is trying to speed up diplomatic clearance. “[At the end of May] I had a regular Baltic defense ministers’ meeting with my Latvian and Lithuanian colleagues, where we also discussed how to achieve progress in making the Allied troops movement in the Baltic region simpler and faster,” Estonian Defense Minister Hannes Hanso told me. “We agreed to continue the work on simplifying the relevant procedures and regulations. Estonia has already eased its regulations about requesting the entry permissions for military vessels and aircraft.” According to the new regulations, the permission must be granted within seven working days, though Hanso says that in practice the procedure usually takes only a few days, and in urgent cases the permit can be issued within hours.

NATO Secretary-General Jens Stoltenberg briefs the media ahead of a NATO foreign ministers meeting at the Alliance headquarters in Brussels, Belgium, May 19, 2016. Francois Lenoir / Reuters Raimonds Bergmanis, Latvia’s minister of defense, makes a similar argument. “A critically important aspect of the Baltic Sea Region deterrence is the swiftness of movement to obtain maximum effect by rapidly deploying forces of high readiness,” he told me. “Currently all three Baltic states are working on simplifying procedures and regulations in order to facilitate movement and cross-border activities of the Allied troops, aircraft, vessels, and their equipment in the region. The aim is to bring all regulations in line with the concept of the VJTF and to enable deployment at a very short notice.” In Latvia, Bergmanis adds, NATO aircraft enjoy multiple-flight permits, and NATO warships don’t need diplomatic clearance.

But military commanders, hoping for more progress—and more uniform progress across Europe—are arguing for an EU-inspired military Schengen. The Schengen Agreement, in place since 1996, allows passport-free passage between the 28 European countries that are part of the arrangement. (Of the EU’s member states, only the United Kingdom and Ireland have opted out, and several relatively recent EU members, including Romania, are waiting to join.) Hodges, to build the case, carries with him a chart showing the logistical challenges of transporting troops and equipment that he shows to decision-makers. The chart, classified for government use only, shows nine of NATO’s 28 member states, including Poland and Turkey, requiring 15 days or more for diplomatic clearance.

With a military Schengen in place, NATO troops and equipment would be able to cross NATO borders to their destination the same way EU citizens do: without having to show permits.

Should a war break out, SACEUR Curtis Scaparrotti and his fellow NATO commanders would, of course, be free to move their troops across NATO borders without diplomatic clearance. “All nations have signed up to these crisis-response measures, so we don't expect any hold-ups in a crisis,” says the NATO spokesperson, adding that “an early entry force will be able to deploy within a couple of hours, supported by the national forces and the VJTF.” But many commanders and analysts, including Brezinski, argue that peacetime red tape is affecting planning and preparations for such contingencies, which in turn affects deterrence. “Only by regular exercises involving the movement of large numbers of troops and equipment can you minimize the logistical grit and confusion that inevitably accompanies the fog of war,” Brzezinski told me.

Indeed, when it comes to logistics, the lack of a military Schengen may put Russia at an advantage over NATO. “The Russians can quickly move their troops around inside Russia,” notes Hodges. “We're surprised every time they do a snap exercise, and we can't surprise them with snap exercises of our own.”