Ukraine Needs a ‘Wet Gap Crossing’ to NATO

The Biden administration sometimes refers to the need to build a “bridge” to NATO membership for Ukraine. It’s an apt metaphor—just not in the way its proponents might think.

One might think of a bridge as a mere symbol of hope. But, invoked in a military context, a bridge is best understood in its role as wartime infrastructure. And that metaphor works precisely because building a bridge in wartime is an incredibly difficult and complex operation—one that military planners call a “wet gap crossing.” Conducting a contested wet gap crossing is perilous—see Ukraine’s evisceration of a Russian battalion attempting to cross the Siverskyi Donets River in May 2022—but the possible strategic rewards are high. In 1944, George S. Patton’s Third Army crossed the Moselle River at Nancy, turning the German defensive line and opening a strategic position for the Battle of the Bulge.

Much like a wet gap crossing, bringing Ukraine into NATO would be risky and costly, but it could lead to strategic success. If NATO nations are truly serious about bringing Ukraine into NATO, then creating a bridge to NATO cannot just be a clever diplomatic metaphor, and it should not be attempted merely in order to get to the other side, like the Russians at Siverskyi Donets. It has to be approached like the difficult, sophisticated, multifaceted operation that it is, and it must be part of a broader strategy for postwar Euro-Atlantic security, as was the Moselle crossing in World War II.

Diplomats and politicians planning for Ukraine’s future role in NATO at July’s NATO summit in Washington would do well to understand the U.S. military’s own approach to wet gap crossings. The lessons are instructive—and sobering.

Step 1: Try to go around

Because wet gap crossings are so difficult, the preferred option, if possible, is to avoid them altogether. Some would say we should not bring Ukraine into NATO because it is too risky. But that ignores the fact that there are no good options short of NATO membership for Ukraine, and the risks of not bringing Ukraine into NATO are greater in the long run. As in military operations, crossing a river often is the fastest, most effective way to an objective.

Despite the known risks and difficulties inherent in combat bridging, militaries still maintain this capability because they know that sometimes the strategic opportunity afforded by a successful wet gap crossing is worth the risks and difficulties. They also know that sometimes, going around is not an option. Russia has invaded its neighbors and rattled its nuclear saber, but one thing it has not done is attack NATO directly. That is because NATO’s Article 5 remains an effective deterrent. Nothing else has worked.

Those arguing against Ukrainian membership in NATO assert that perhaps we should choose an “Israel model” of continued materiel support to Ukraine or that a combination of countries, such as the G-7 nations, providing long-term economic support to Ukraine, would convince Russia that it cannot win. The Israel model will not work because Israel has nuclear weapons and Ukraine does not. In fact, that’s the whole point. Ukraine gave up its nuclear weapons in 1994 when Russia, among other nations, agreed to respect Ukraine’s sovereignty and territorial integrity. Similarly, Sweden’s and Finland’s decisions to join NATO despite already being members of the European Union demonstrate that bringing Ukraine into the EU and affording it the EU’s Article 42.7 mutual assistance clause would be insufficient to deter Russian aggression.

Step 2: Plan and rehearse

Once a decision has been made to conduct a deliberate wet gap crossing, planning is crucial. Simply moving your forces up to the edge of the water and trying to figure out a way across when you reach it would guarantee disaster. You must reconnoiter potential crossing sites, assess which will likely be successful given the terrain as well as your and your enemy’s strengths and weaknesses, and prepare multiple crossing sites.

There are several options for bridging Ukraine into NATO, all of which should be considered but not all of which seem promising. The first—declaring Ukraine a NATO member while hostilities are ongoing—is theoretically possible but likely politically untenable given the need for unanimity among the 32 allies to bring in a new member. The fact that it took a year to bring the geographically blessed and militarily advanced Sweden into the alliance belies this harsh fact. If, somehow, this became politically tenable, then NATO would have to quickly deploy forces into Ukraine to make the Article 5 guarantee more than just lip service.

The second option would be to bring Ukraine into NATO as part of a guarantee during negotiations over a cease-fire or cessation of hostilities—i.e., as soon as a cessation is in place, Ukraine will accede to NATO. This likely would not work because Russia would continue fighting rather than agree to a cessation of hostilities that triggered Ukrainian membership in NATO.

The third option would be for a critical mass of NATO nations to guarantee Ukrainian sovereignty and territorial integrity following a cease-fire by deploying forces on Ukrainian territory. This has the benefit of offering concrete security guarantees to Ukraine while allowing time to bring onside skeptical NATO nations.

While the future shape of Ukraine is unknowable, and the timeline for Ukrainian admission to NATO is unknown, the alliance should start working now to achieve unanimity of political support among NATO nations for Ukrainian accession and also to determine how, where, and when forces from NATO nations will be used to guarantee the sovereignty and territorial integrity of Ukraine. Both measures will be unavoidable, regardless of which option is deemed most credible.

Step 3: Prepare the battlespace

In combat bridging, you don’t just line up all your vehicles in a convoy and drive directly to the location where you want to build your bridge and then start putting things in the water. That would be suicide. You plan, rehearse, prepare your forces, and conduct a preparatory campaign to establish favorable conditions. Similarly, simply declaring a Ukrainian bridge to NATO without doing any planning or preparation would just leave Ukraine in the same strategic limbo it faced following the 2008 Bucharest declaration and similarly would motivate Moscow to redouble its efforts to undermine Ukrainian sovereignty before it is able to join NATO.

For NATO, this means that members need to begin whipping together votes in favor of Ukrainian NATO accession now. Diplomats need to understand who in the alliance already is on board with bringing Ukraine into NATO and under what conditions. For those whose position is “never” or “not until the war is over,” more creative solutions must be proposed, discussed, and solidified—in private. This cannot be a one-off discussion; it must be a constant campaign to prepare the battlespace for eventual Ukrainian accession.

Regardless of whether the war ends with Ukraine in control of its 1991 borders or Kyiv settles for something short of that, troops from NATO nations will need to be stationed on Ukrainian soil to provide the time, space, and security necessary to complete the bridge into NATO. These forces should include a coalition of key allies—ideally including NATO’s three nuclear states (Britain, France, and the United States) to signal that despite a lack of Article 5 security guarantees, NATO’s nuclear nations are committed to upholding the agreed-on borders—just as NATO troops were stationed in West Germany to deter Soviet forces in East Germany in the years between the end of World War II and West Germany’s accession to NATO.

Moving these forces into Ukraine in a short timeframe following an armistice or cease-fire would be extremely difficult both logistically and politically. Therefore, NATO nations should begin to set the theater now for those moves by declaring that NATO’s air defenses surrounding Ukraine will begin to shoot down Russian missiles and one-way attack drones that are on a trajectory to hit NATO territory; sending small numbers of NATO military personnel into Ukraine to provide training to Ukrainians; and negotiating with Turkey on allowing NATO naval capabilities into the Black Sea to protect civilian shipping.

Step 4: Commit

A wet gap crossing is a massive operation. It is viewed as a corps-level effort in the U.S. Army and is assumed that the Air Force, Space Force, and cyber assets also will provide critical support. It is difficult, risky, and costly, but if done properly, it can lead to strategic breakthrough.

Precisely because it is so risky, the commander of the operation must assess the risks involved, mitigate as much risk as possible without jeopardizing the mission, and accept that it is impossible to mitigate every risk. This is a critical step because once a combat wet gap crossing has begun, a commander must fully commit to the plan and leverage all forces available to make it a success. Half-measures in this type of operation lead to failure.

If NATO is serious about bringing Ukraine in as a member—and it should be—then it must be clear-eyed about the risks. It must develop a concrete plan, not just a political laundry list. This plan must be in support of a broader strategy. And most importantly, it must commit itself to success. Anything less is likely to lead to failure.