Before Russia's full-scale invasion in February of 2022, there was no fence in major parts of the border between Ukraine and Russia. But in the two and a half years since then, Ukraine has tightened security, not to keep Russians out, but Ukrainian men in. In tonight's Prime Focus, ABC's Patrick Reville is on the ground in Ukraine, reporting on the depletion of its most precious resource, manpower, and the dwindling numbers of enlisted personnel taking the fight to Russia.

Barbed wire on Ukraine's border. But the barriers aren't designed to keep Russians out, but rather Ukrainian men in. Since Russia's full-scale invasion in 2022, Ukraine has barred most military-age men from leaving. As a result, the number of attempted illegal border crossings has soared.

We're on Ukraine's western border, and just over there is Romania, just on the other side of the river. People have been trying to make this crossing, and it's dangerous. People have died. The water is very fast flowing. It's very cold.

This video showing guards pulling one man from the river, not moving. Recently, security here has tightened again because of a hard truth. Ukraine is short of soldiers. After For more than two years of brutal warfare, Ukraine's forces are dangerously depleted. Soldiers on the front line are exhausted, and in some places are now increasingly heavily outnumbered by Russia, sometimes by 7 to 10 times, according to Ukraine's military. Even more than weapons, most analysts say Ukraine now needs more troops. At the start of the invasion, a gigantic wave of Ukrainians volunteered to fight, but that enthusiasm has faded. Now, conscription officers scour the streets trying to find draft eligible men, many who are now in hiding. Videos like these have multiplied. Officers grabbing men on the street. Authorities saying they are investigating this incident. Local recruitment officials say this man resisted being sent to basic training after being conscripted in the city of Chenevci. Officers forcing him onto a bus. Scuffles breaking out. Bystanders filming this incident in Kyiv, where people try to stop officers taking a man. Police insisting such incidents are rare, but the issue driving a painful wedge in Ukrainian society.

Because the young guys, by now, they're not motivated. The people want to protect our citizens or our homes. But by now, probably not all..

It's not that Ukraine doesn't have enough men, but for many months, the government was reluctant to conduct a new mobilization, knowing it was unpopular. At the border, many men trying to cross illegally are not even yet subject to the draft.

But Ukraine's government is now finally calling up more men.

In May, a new mobilization law came into force, tightening conscription rules and lowering the draft age from 27 to 25. Since then, more men have tried to leave the country. Ukraine's border service US releasing this video recently showing men it says were stopped trying to sneak out illegally. Some of those who leave say they fear being sent to fight without proper training. This man left Ukraine last year with his family. We are disguising his identity over fears he could face repercussions for speaking.

To be honest, it's very complicated and hard profession to be a soldier. You have to have been trained for many years to be an efficient soldier, not just cannon father. I think that I almost have no chances to survive if I will be sent by soldier just in close combat.

Do you worry that Ukraine will lose the war because it doesn't have enough men to fight?

Well, of course, I worry. I worry about it. Of course, I do. I actually continue to pay taxes in Ukraine, and I make the donations on a monthly basis to different funds. But I'm not sure that they I will be a good soldier, and I'm not sure if I die, as I said in first five minutes, it can help for my country to will. Maybe, maybe. But I'm not sure about that.

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But there are still those who are willing to volunteer to fight. These men are going off to war. From this train station in Kyiv, they're bound for three months of basic training with a well-known volunteer battalion, the Da Vinci Wolves.

You think you have no choice but to volunteer now?

I have choice, and that's my choice. I think it's the choice everybody makes for themselves. Nobody took me by my clothes and dragged me here.

Most of the men have no military experience. They've joined this battalion in part because it's known to offer better training and command. Pavelo's wife, Ludmilla, has come to see him off. Analysts say the new draft in Ukraine will likely begin to have an impact later this summer. But what's clear is Ukraine will many more goodbyes like this before the war ends.

Such a difficult time for so many families there. Our thanks to Patrick.