Ukrainian soldiers suspected they would invade Russia when they got new rifles, but still thought it was a joke when the orders came through

• Ukrainian's shock offensive on Russia's Kursk region came as a surprise even to Ukraine's soldiers.
• "We joked that it wasn't April 1st," a Ukrainian soldier told The Economist .
• The country's troops did suspect that an invasion was imminent after they were issued new equipment.

Ukraine's plans to attack Russia's Kursk region were kept so under wraps that its own soldiers didn't believe it was going to happen.

Ukrainian soldiers started to suspect that an attack was imminent after they were issued new helmets and assault rifles earlier this month, The Economist reported on Sunday, citing soldiers involved in the incursion.

The soldiers told the outlet they went through training exercises on mock-ups. They later realized the mock-ups were simulations of Russian villages.

But for some, invading Russia still felt like a distant possibility.

"We laughed," a Ukrainian soldier identified only by his first name, Serhiy, said of the plans when he first heard of them, per The Economist.

"We joked that it wasn't April 1st. The commander just smiled, knowing we had no idea what was awaiting us," said Serhiy, a private in Ukraine's 80th brigade.

Concealing the invasion plans until the very last moment proved to be a masterstroke for Ukraine's military, whose shock offensive into the Kursk region on August 6 allowed them to catch the Russians off guard .

The commander in chief of Ukraine's military, Oleksandr Syrskyi, said last week that they'd seized nearly 400 square miles of Russian territory in just a few days. That's almost as much territory Russia has seized from Ukraine this year.

Ukraine's recent gains on the battlefield mark a shift in fortunes for the embattled country, which was caught on the back foot after US Congress delayed more than $60 billion of aid to them. The aid bill was finally passed in April after months of GOP opposition.

To be sure, Russian complacency might have added to the effectiveness of Ukraine's clandestine military operation.

Andrey Gurulyov, a Russian lawmaker and retired major general, said in a televised interview in Russia on August 8 that the country's military knew about Ukraine's plans to attack Kursk a month before it happened .

"But from the top came the order not to panic, and that those above know better," Gurulyov said, according to a translation by The New York Times .

Representatives for Ukraine's defense ministry didn't immediately respond to a request for comment from Business Insider sent outside regular business hours.