AMNA NAWAZ: President Trump has ramped up his criticism of Germany's leader after Chancellor Friedrich Merz said the United States had been humiliated by Iran.

In a post on TRUTH Social, the president told Merz to -- quote -- "spend more time on ending the war with Russia and Ukraine, where he has been totally ineffective, and fixing his broken country."

Despite Trump's rhetoric and repeated threats to withdraw the U.S.

from NATO, Germany's top military officer claims relations with American military leaders are as strong as ever.

But he also says the threat from Russia means Germany's push to strengthen its own armed forces is a race against time.

Special correspondent Malcolm Brabant reports from Germany.

MALCOLM BRABANT: In Marienberg in former East Germany near the Czech border, the country's newest recruits perform a last-minute drill before their passing out parade.

For some, the army offered a path out of unemployment.

Others were motivated by Ukraine's resistance against Russia.

For security reasons, they only gave their first names.

FELIX, Army Recruit (through translator): My personal view is that I owe a lot to this country, for education, the health system, and I would call myself a patriot.

JULIA-MARIE, Army Recruit: Of course, I think I'm a little bit concerned, but I'm not really afraid that Russia will attack us or a war will break out, because I think it's more like a warning or a wake-up call for us.

MALCOLM BRABANT: Marching alongside Marienberg's town band, the 371st Mechanized Infantry Battalion will become the tip of NATO's defensive spear.

They can expect to be deployed to Lithuania to strengthen the alliance's eastern flank.

If the German government gets its way, passing out parade is going to be a far more frequent occurrence.

The German chancellor, Friedrich Merz, has said he will do whatever it takes to make sure that Germany is war-ready by 2029.

He's boosted defense spending to make sure that by the end of the decade, Germany will have the strongest military force in Europe.

This footage from the Bundeswehr, or federal armed forces, shows mechanized infantry training to defend the West against Russian aggression.

How dangerous would you say this particular period is?

GEN.

CARSTEN BREUER, German Chief of Defense: I'm now in the military since 42 years, and I must say I have never experienced such a dangerous situation like we are having at the moment.

The clock is ticking.

MALCOLM BRABANT: General Carsten Breuer is Germany's most senior military officer in charge of the Bundeswehr, comprising the army, navy and air force.

GEN.

CARSTEN BREUER: Russia is rebuilding their forces.

We see that the Russian personnel, military personnel is going up to 1.6 million soldiers.

So what we see is a restructuring of the Russian military, and it's all directing against the West.

MALCOLM BRABANT: Increasing the pace of Germany's free armament has become more urgent after President Trump reiterated his dismay over America's NATO allies' refusal to join the war against Iran.

DONALD TRUMP, President of the United States: We spend hundreds of billions of dollars a year on NATO, hundreds protecting them, and we would have always been there for them, but now, based on their actions, I guess we don't have to be, do we?

MALCOLM BRABANT: While Germany's relationship with the White House may be turbulent, according to General Breuer, connections with his American counterparts are secure.

GEN.

CARSTEN BREUER: Are there any breaks in military relations?

Definitely not.

They are as good as in former times.

It's totally clear and we also understand that we have to take over more responsibility for our own security.

MALCOLM BRABANT: Germany's rearmament alarms the nation's teenagers; 18-year-old males are now required to declare whether they're willing to serve.

Fears that compulsory conscription will follow are driving widespread school strikes and marches.

OLE NYMOEN, Author: I don't want to fight in a war for Germany.

I don't want to die for Germany.

I don't want to kill for Germany.

MALCOLM BRABANT: The anti-war rhetoric of author and podcaster Ole Nymoen has turned him into a national figurehead.

OLE NYMOEN: The state doesn't care whether people want to fight or not.

It's just forcing its citizens to.

And this is, in my opinion, the most important argument against fighting for your country.

MALCOLM BRABANT: Stroll through the German capital, and it's easy to find evidence of the decisive 1945 battle between victorious Soviet troops and Berlin's defeated defenders.

Pillars on Museum Island are pockmarked with bullet holes.

As it atoned for its Nazi past, Germany has projected a peaceful posture.

Now the Bundeswehr is having to counter deep-rooted skepticism about its purpose.

The job of Captain Mike Siebert is to convince young people that the army is an essential pillar of Germany's democracy.

He's taking a class on a tour of Parliament.

CAPT.

MIKE SIEBERT, Youth Officer: As the information officer, our job is political education in terms of security policy.

And we are institutionally prohibited from any kind of recruitment.

So if we get invited for a teacher and they're requesting a study trip, for example, it's good that the students get a chance to see that democracy is not abstract.

MALCOLM BRABANT: Opinion polls have consistently shown that most young Germans oppose conscription and are unwilling to fight.

A contrast was provided by this predominantly middle-aged audience at a debate about choosing freedom, who were asked to stand if they'd take up arms to defend Germany.

So will the military reach the required numbers to counter the Russian threat?

Sonke Neizel is a military historian.

SONKE NEIZEL, Military Historian: There are definitely enough young German males who want to defend Germany.

It's about 12 percent.

And you could argue, well, it is not enough, but we don't need any size of an army of millions of soldiers.

We are aiming for 260,000 and possibly, unofficially, up to 300,000.

But this is enough.

MALCOLM BRABANT: In common with other nations which have donated equipment and ammunition to Ukraine, Germany's stocks are severely depleted.

Lieutenant Colonel Marcel Bohnert represents the German Armed Forces Association.

LT.

COL.

MARCEL BOHNERT, German Armed Forces Association: So ammunition is one of our biggest challenges.

So, we need much more.

Our industry builds up capacities currently, so they will produce a lot more.

But we are only able, I guess, to defend Europe with our partners.

So only munitions of Germany will not be enough to really, yes, defend us.

MALCOLM BRABANT: As Russia triples its defense spending, drone warfare is one area of Germany's military-industrial complex that needs rapid growth.

STARK, a small start-up, has won a $300 million contract for its loitering suicide drone that's been tested on the front lines in Ukraine.

Senior vice president Josef Kranawetvogl shows off the warhead that can take out a tank.

JOSEF KRANAWETVOGL, Senior Vice President, STARK: It's highly effective, can destroy up to 800 millimeters of armored steel.

It's battle-proven from beginning on, I would say, because we get quite a lot of feedback from soldiers that use our systems from the front line.

It is essential that you scale up your production right now and it can't be done within some minutes or seconds, so it needs some time to ramp up production and to set up production sites.

And we are on it to do this right now.

MALCOLM BRABANT: Germany's race to build its stockpile of weapons and ammunition is predicated on the fear that in the not-too-distant future an emboldened Russia may test NATO's strength by attacking one of the former Soviet Baltic states.

GEN.

CARSTEN BREUER: Is it a threatening situation at the moment?

Is it a dangerous situation at the moment?

Yes, it is.

Are we going into the war?

No, we don't.

Because we have understood this, we are preparing to defend ourselves, and, therefore, deterrence is key.

Deterrence is coming back, deterrence for NATO, deterrence for European countries.

And if you have this deterrence, this is your insurance.

MALCOLM BRABANT: In Marienberg, the young grenadiers swore to faithfully serve the federal republic and to bravely defend the rights and freedom of the German people.

ROBERT, Army Recruit (through translator): It's important for me to stand up for my country, because this country gives you so many possibilities, so many freedoms.

MALCOLM BRABANT: Brave words on such a big day, but their families are hoping that the vow of their sons and daughters won't be put to the test.

For the "PBS News Hour," I'm Malcolm Brabant in Marienberg.