People are flocking to Ukraine to take on the Russians. Here's what foreign vets who've done it say that fight is like.

• Ukraine has received considerable military aid and training from Western militaries.
• Now more people from those countries are now heading to Ukraine to fight the renewed Russian invasion.

Russian military commanders invading Ukraine knew that their Ukrainian opponents had received significant training and weapons from the US and other Western countries.

Reports about the potential use of US-made FGM-148 Javelin anti-tank missiles and FIM-92 Stinger anti-aircraft missiles began surfacing as Russian President Vladimir Putin was amassing hundreds of thousands of troops along Ukraine's borders with Russia and Belarus.

More informed Russian military commanders also knew that in addition to the Ukrainian military and national guard, they could end up facing small groups of foreigners — mostly Westerners — who had gone to Ukraine to join the fight against Putin's aggression.

Ukraine's foreign legion

These foreign fighters first joined the Ukrainian military after Putin invaded and seized the Crimean Peninsula in 2014. An insurgency in the Donbas region of eastern Ukraine began at about the same time, as Russian-backed separatists in the region sought to break away from Kyiv's rule.

Foreign fighters have gone to Ukraine to advance both pro-Ukraine and pro-Russian causes; right-wing extremists have also been drawn to both sides of the conflict. Ukraine's government has set standards for foreign volunteers and tried to screen those coming to fight.

"For us, this is the same war that began in 2014. After the quick annexation of Ukraine, public interest faded away. Besides the occasional reporting by the mainstream media, people didn't pay attention to the war in Ukraine," a former US Marine who fought with the Ukrainian military in Donbas told Insider.

There was "an uproar" after Malaysian Airlines Flight 17 was downed by what investigators said was a Russian-made missile over territory held by pro-Russian separatists in eastern Ukraine, "but even that didn't last for long," the former US Marine said.

Joining the Ukrainian military or working with them as an advisor was a way for American and British veterans of the war on terror to remain relevant, but the choice to go to Ukraine was and is an individual one.

"It was a noble fight. That's what drew me there. Everyone had his own personal reasons for going," the former Marine said. "Some were after money. Others went because of their ideology. Some wanted to get an adrenaline 'fix' and continue to feel relevant in a society that had left them behind. Everyone went for his own reasons. For me it was a chance to fight in a noble war. I was helping defend a smaller nation from the invading forces of a larger bully."

Ukrainian President Volodymyr Zelenskyy has called for foreign volunteers to join Ukraine's military, and the renewed invasion has prompted many foreigners to flock to Ukraine to fight the Russians.

Ukraine's foreign minister said Wednesday that more than 1,000 volunteers from 16 countries were on their way, and a Ukrainian defense official told Military Times that Ukraine's Embassy in the US had received more than 3,000 applications from US citizens who want to join the fight.

Several European nations have authorized their citizens to go to Ukraine and answer Zelenskyy's call to arms. US officials, however, have cautioned Americans against going there.

"What we would tell Americans who want to help Ukraine is to find ways to donate and contribute to the many nonprofits that are already working to try to get humanitarian assistance into the people in need," chief Pentagon press secretary John Kirby said Wednesday.

Putin: Ukraine's best recruiter

The Ukrainian military and national guard have so far put up much fiercer resistance than Russia and the world expected.

"The Ukrainians are really good. They might lack the weapons and tech we enjoy in our Western militaries, but they have heart. You can never go wrong with heart," a former British Royal Marine who fought in Ukraine told Insider.

"If this comes down to an insurgency, heart will carry them through much better than high-speed rifles or gear," the former British Royal Marine said.

The former US and British Marines spoke on the condition of anonymity to protect their identities and those of who they fought alongside in Ukraine. Veterans of the fighting there who spoke to Insider also acknowledged the capabilities of Russian troops and the separatists they back.

"The Russians are good. My take on why they are underperforming in Ukraine now is that they weren't ready for it," the former US Marine said. "It's one thing to do large-scale exercises to show off your military power and another to invade a brotherly nation (especially in eastern and southern Ukraine where it's like you're in Russia). I think the Russian grunt is as surprised as the world for this invasion."

US officials and experts on the Russian military have also said Russian troops seem surprised or unprepared for the operation and that the tactics on display differ from those Russia would be expected to employ.

"I only have experience with the [Russian-backed] rebels. They are savvy fighters who know how to exploit your vulnerabilities. With help from their Russian friends, they figured out how to combine information warfare and military operations," the former Royal Marine said, describing an incident where rebels used a cellphone call between a Ukrainian commander and his mother to target a strike.

"They knew that he wouldn't get on the phone for any other reason, so they called his mom and pretended to be Ukrainian and told her that her son had been wounded," the former Royal Marine said. "His mom immediately called him on his personal phone, and they talked briefly. But that was enough to get a geolocation for an artillery strike."

Russian forces struggled to advance in Ukraine in the first days of the invasion, during which they appeared to try to avoid deliberate targeting of Ukrainian civilians, but many civilians, including children, have already been killed, and experts expect Russian attacks to get less discriminate as frustration mounts over their lack of progress.

"It is definitely sad to see the human loss of life" and the destruction of Ukrainian cities, the former US Marine said. "I can't help feeling we let Ukraine down. For the Ukrainian people and for us fortunate to have lived with them, this war was long in the making. I feel sorry about how events played out, but I saw it coming."

Stavros Atlamazoglou is a defense journalist specializing in special operations, a Hellenic Army veteran (national service with the 575th Marine Battalion and Army HQ), and a Johns Hopkins University graduate.