Six weeks before a critical summit meeting aimed at bolstering NATO’s deterrence against a resurgent Russia, the alliance is facing a long list of challenges. The first is to find a country to lead the last of four military units to be deployed in Poland and the three Baltic nations.

But that, analysts say, could be the least of its problems.

Security concerns are as high now as they have been since the end of the Cold War. As the immigration crisis has strained relations within the Continent, anxieties have been heightened by Russian military offensives in Crimea and eastern Ukraine, and a bombing campaign in Syria that has demonstrated Moscow’s rapidly increasing capabilities. Lately, Russia has talked openly about the utility of tactical nuclear weapons.

Despite the growing threats, many European countries still resist strong measures to strengthen NATO. Many remain reluctant to increase military spending, despite past pledges. Some, like Italy, are cutting back. France is reverting to its traditional skepticism toward the alliance, which it sees as an instrument of U.S. policy and an infringement on its sovereignty.

And that is not to mention the declarations of the presumptive Republican presidential nominee, Donald Trump, that NATO is “obsolete,” that the allies are “ripping off” the United States and that he would not really be concerned if the alliance broke up. While that may be campaign bluster, it does reflect a growing unwillingness in the United States to shoulder a disproportionate share of the NATO burden, militarily and financially.

The current concern, and a major element of what the NATO secretary-general, Jens Stoltenberg, calls “the biggest reinforcement of collective defense since the end of the Cold War,” is the decision to put four combat battalions of up to 1,000 soldiers each in those front-line countries bordering Russia.

While Britain, Germany and the United States have agreed to lead one battalion each, to be filled out with soldiers from other NATO allies to preserve the idea of multinational forces, leadership of the fourth is not yet in sight as the July 8-9 summit meeting in Warsaw rapidly approaches.

The United States “is not thinking about doing two,” said its ambassador to NATO, Douglas E. Lute. “We’re planning to do one and get our allies to step up” for the other three.

But other larger nations like Italy and France have declined. Italy cut military spending after pledging to increase it two years ago in Wales. Its leaders say it is already participating in a newly enlarged alliance rapid-reaction force.

And France, which has reverted under the current Socialist government to a more mistrusting view of NATO and its U.S. leadership, is stretched thin in its military campaigns in Mali, the Central African Republic and North Africa and Syria, let alone patrolling its own streets against terrorist attack.

France is likely to contribute only about 150 soldiers to the new deployments, NATO officials say, after finally agreeing to the idea of forward deployments in Poland after initial opposition. Germany, which six months ago opposed these deployments, agreed in return for efforts at renewed dialogue with Russia. It also agreed to lead one battalion.

So the search goes on for a fourth lead nation. Stoltenberg is confident it will be found by the summit meeting.

The deployments are important, because these combat battalions are designed not to be simple tripwires, but to be large enough and sufficiently well equipped to do an invader real damage. Then they can be reinforced more quickly with the enhanced rapid-reaction force — another NATO and American decision is to station an added U.S. armored combat brigade of around 5,000 soldiers in Europe (for a total of three) and to preposition its heavy equipment like tanks and artillery.

Poland is demanding that some of that equipment be prepositioned on its territory, but for the moment, most of it will go to Germany, Belgium and the Netherlands, which have storage and transport facilities dating from the Cold War.

Only now, in fact, is NATO actually surveying the infrastructure — the bridges, roads and railways — of relatively newer member states in Central and Eastern Europe, not having judged it necessary before to plan how to quickly reinforce them in case of a Russian invasion. Prepositioning in Eastern Europe would currently require large sums for capital investment to build special new warehouses and infrastructure, Lute said.

Poland, eager to send messages to Moscow, did succeed in advancing groundbreaking for a ballistic missile defense site to coincide with the operational opening of one in Romania. While Stoltenberg and Washington insist that such missile defenses are not aimed at Russian intercontinental ballistic missiles, Moscow is not convinced.

Here, too, France has been skeptical, nervous that the reaction time of missile defense will circumvent the political oversight of the North Atlantic Council, the assembly of member states and their ambassadors that makes NATO decisions by consensus.

For the same reason, France has been reluctant to allow the NATO supreme commander, who is always a U.S. general, too much authority to act in a crisis, which others say is needed to respond quickly.

But the initial missile defense program, combined with new forward deployments near Russia (to be rotated, to avoid calling them “permanent”) and an enlarged rapid reaction force, supposedly ready to deploy in 48 hours, all are a measure of how much Russia’s recent actions have changed NATO’s calculations.

NATO is trying to reassure vulnerable members like the Baltic States, Poland and even southern members, like Romania, Bulgaria and Turkey on the Black Sea, that the alliance intends to deliver on its promise of collective defense. Missile defense is part of the response, along with more naval exercises in the Black Sea and more consistent overflights by reconnaissance aircraft.

As Stoltenberg points out, the effect of Russian policy has finally pushed European members of NATO to at least halt the decades-long decline in military spending. This year, he said, estimates are that European allies will as a whole increase military spending, something the U.S. has been demanding, even though most are not yet spending the 2 percent of GDP that is the NATO guideline.

Some 16 of the 28 member states have increased military spending in real terms, with only Italy, Bulgaria and Croatia still cutting, although they insist that the cuts are temporary.

“I know the mood in Washington and I understand it: the Americans want to see the Europeans doing more, contributing more,” Stoltenberg said. “This has been my main message in European capitals.”

Still, there is another troubling Russia-related issue for NATO — how to deal with a new Russian military doctrine that considers the utility of tactical nuclear weapons at the beginning of a conflict, as a deterrent against an adversary retaking territory, followed by what planners call “a quick de-escalation.”

Some member nations believe that Russia already has nuclear weapons in the enclave of Kaliningrad, on the Baltic Sea, where it publicly displayed nuclear warheads in a previous exercise. Russia has been unclear about whether they have been removed.

While aware of public horror about using nuclear weapons, Stoltenberg, Lute and others emphasize that NATO “remains a nuclear alliance” and that its deterrence is meant to be “seamless,” ranging from responses to cyberattacks through conventional weapons and if necessary, nuclear weapons, too.

NATO does not regard its nuclear arsenal as having any use other than political deterrence, Stoltenberg said. “But as long as nuclear weapons exist in the world,” he said, “we have to remain a nuclear alliance.”

© 2016 The New York Times Company