AMNA NAWAZ: Tonight, we go inside Ukraine's counteroffensive.

The U.S. has sent Ukraine tens of billions of dollars of weapons, and senior U.S. military officials predict Ukraine has only about a month to make progress before rainy weather makes movement difficult.

The counteroffensive's primary goal is in the south, toward the city of Melitopol, to cut into Russian-occupied territory north of Crimea.

With the help of the Pulitzer Center, Nick Schifrin, videographer Eric O'Connor and their team spent a week on the front line and report from south of the town of Orikhiv.

NICK SCHIFRIN: At the epicenter of an existential war, Ukrainians try to move forward by any means necessary.

They fire an anti-tank rocket-propelled grenade unconventionally at Russian troops a mile-and-a-half away.

This is a rare look at Ukraine's special Artan Special Forces Unit.

The commander's call sign is Zachar.

He is named for a Ukrainian warrior, but apparently knows American movies.

What's your message to America?

ZACHAR, Artan Special Forces Unit (through translator): What's the message?

If you want to be free, help us with shells, shells and weapons.

The rest, we will do ourselves.

NICK SCHIFRIN: They are the counteroffensive's tip of the spear.

Their intelligence comes from a drone that flies over Russian lines and provides real-time feedback to adjust their fire.

Down the road, just after daybreak, that drone unit brings us to their hidden position.

It is an active morning.

This is right at the epicenter of the counteroffensive, where Ukrainians have pushed the Russians back a little.

They're trying to expand their territory.

The Russian line is just half-a-mile both that way and that way, and you can hear all the firing.

This is the meeting of past and present that helps define this war.

Soldiers walk through the kind of trenches that troops have used for two millennia to be able to launch the most modern form of warfare that Ukraine is helping to invent.

They're from the National Guard 15th Operational Brigade and the Makhno Unit, a special forces team named for a Ukrainian revolutionary born more than a century ago.

But it's not just reconnaissance, explosives on top, a charge on the bottom, and a drone in the middle that will be flown as a kamikaze.

Their first step is a surveillance drone that will help guide the kamikaze pilot.

The Chinese-made surveillance Mavic drone flies toward the Russian position.

The kamikaze drones are called FPVs for first-person view from video goggles.

The pilot, call sign Maki, races the drone toward Russian positions, the surveillance drone helping guide him in.

And then they see their target.

Maki flies the kamikaze FPV across no man's land until he loses connection the moment it explodes in a Russian trench.

MAKI, Makhno Unit (through translator): I (EXPLETIVE DELETED) up in the trench.

The trench is a straight line and I was trying to hit behind him.

I flew like that and the fragments flew all over the trench.

NICK SCHIFRIN: After, the hunters become the hunted, and they take cover from Russian drones.

Another surveillance team searches for Russian positions.

Those craters are from artillery shells.

Call sign Ahmet explains how a network of drones feeds intelligence into a live interactive map.

AHMET, Ukrainian Soldier (through translator): You can indicate on the map what you see, artillery, infantry and equipment.

And everyone will have access to the map.

NICK SCHIFRIN: They say a platoon's life can hinge on a single drone.

And so the two sides try to blind the other.

The unit's connection depends on this Starlink satellite.

They couldn't fly for more than an hour because nearby Russian troops jammed the signal.

AHMET (through translator): If they are two to three kilometers away, I can fly the drone and see how our artillery is working.

The same with intelligence.

I can take a photo, we can sit and plan, and the group will already know where there's a foxhole, where there's a dugout.

They will not go in darkness.

But this advantage is not only for us.

It's also for them.

NICK SCHIFRIN: Since June, Ukraine has pushed Russian troops back a few miles deep and a few miles wide here in blue past Robotyne, and, in recent days, near Verbove.

The short-term goal is to push further toward Tokmak and sever the Russian land bridge toward Crimea.

But Russia has reinforced positions to the south.

The job of protecting one of the Ukrainian flanks falls on this unit of the 128th Brigade artillery here at Forward Base Taxi Driver.

There's a lot of waiting in war, but also contests of strategy, challenges of strength, and even hand-to-hand combat.

You're holding your phone, and you have the messaging application Signal open.

Why?

EUGENE DYACHENKO, Ukrainian Platoon Leader (through translator): There is an artillery group where we get targets and, at any moment, we will need to hit it immediately.

That's it.

There's no other way.

NICK SCHIFRIN: Thirty-four-year-old platoon leader Eugene Dyachenko is from this region, but his hometown is occupied.

His grandmother and brother still live there.

EUGENE DYACHENKO (through translator): They are being terrorized.

The Russian intelligence services come and take them to the basement and ask about me, where am I?

I ask them to keep a low profile and just to wait.

We will be there soon, really soon.

NICK SCHIFRIN: The 128th Brigade is one of Ukraine's most experienced units, and last month liberated the occupied village of Piatykhatky.

Despite that success, despite fighting with 21st century tools, his unit's munitions feed a weapon twice his age, the D-20 howitzer.

Ukraine fires more than 90,000 shells a month.

The D-20 is a Soviet era workhorse that soldiers rely on and arm with help from foreign supporters.

But this melting pot of munitions isn't enough.

At the beginning of the war, they fired 200 a day.

Now they have a fraction of that.

LT. COL. OLEKSII KOVALYUK, Ukrainian Armed Forces (through translator): We are ready to work almost around the clock, just changing shifts, but we don't have ammunition.

NICK SCHIFRIN: Lieutenant Colonel Oleksii Kovalyuk is the commander of the 128th Brigade's artillery division.

You have had to ration your firing.

Have you seen the same thing on the Russian side?

LT. COL. OLEKSII KOVALYUK (through translator): Yes, not like we have, but yes.

There is a decrease in the use of and intensity of their artillery.

It's the same for them as it is for us.

NICK SCHIFRIN: Ukraine's shortage exacerbates the challenge of Russian defenses, hundreds of miles of anti-tank ditches, barriers known as dragon's teeth, an extensive trench network, and the largest mine fields in the world.

Deminers demonstrate how they dig up mines by hand or use specialized vehicles that fire explosive ropes that clear a path to drive through.

This is the view from a drone infrared camera.

Each white spot is a mine, on average, one mine every two square feet for hundreds of miles, often remined from the air.

It's led to enormous losses of Western weapons systems and Ukrainian soldiers.

Ukraine doesn't disclose casualties.

The U.S. officials say the number of wounded and dead since the beginning of the war is more than 150,000.

On the southern front line, medics have never been busy.

All of these videos are filmed by the medics of the 15th National Guard Brigade.

Solomon is one of them.

SOLOMON (15th National Guard Brigade) (through translator): It's hard to say how many there have been, because one dead or one wounded is already too much.

NICK SCHIFRIN: What was this city like before the war?

He walks me through Orikhiv, the largest city near the front, where silence is suspended by sounds of war.

It is a nearly empty shell from bombs dropped by Russian jets, apartment complexes cut in half.

What used to be a school, then aid distribution point, the sole pillar standing still holds children's backpacks, and all that remains of a hospital.

SOLOMON (through translator): This is also a hospital building, and civilians were living in the basement.

The missile hit here.

We see now the result.

There was no military equipment.

For the Russians, there are no rules.

Civilians, soldiers, medics, there is no difference.

And we know from our intercepts that they are intentionally targeting and want to destroy an armored medical vehicle during evacuation.

NICK SCHIFRIN: The 15th fights on this front every day, but neither their soldiers nor their medics have as modern equipment as other regular army brigades.

Have the number of casualties your unit has faced increased because of the minefields?

SOLOMON (through translator): Of course, over the last year, very, very much.

They had time.

They mined very well.

EVGENE (15th National Guard Brigade) (through translator): There are so many minds here, and we don't know about most of them.

NICK SCHIFRIN: Thirty-year-old Evgene is also with the 15th.

The civilian medical infrastructure is obliterated, so military medics are the only source of medical care.

EVGENE (through translator): Civilian ambulances are not coming here, so we help the civilian population.

And it is more difficult, because there is only one road, which is being shelled.

Each trip, there is a lottery whether we will return safely.

NICK SCHIFRIN: And that risk applies to even the most elite units.

That Zachar, commander of the Artan Special Forces Unit, at the funeral of his former commander.

ZACHAR (through translator): Our commander died in Bakhmut.

We were working with a rocket-propelled grenade.

There was incoming, and he was instantly killed.

NICK SCHIFRIN: How will Ukraine win this war?

ZACHAR (through translator): Only victory, only after we return all of our land.

Without all of our land, my belief is that the war will not end.

NICK SCHIFRIN: And so there is no Plan B. Ukraine will keep fighting for every inch, every yard, no matter how long, no matter how hard.

For the "PBS NewsHour," I'm Nick Schifrin south of Orikhiv, Ukraine.