That's a vastly larger cache than Ukraine's collection of wealthier Western allies has managed to pull together.

Ukraine's European allies fell well short of a goal of delivering 1 million shells to Ukraine in the year to May.

Analysts say the North Korean supply line means Russia can sustain its grinding war of attrition in Ukraine for the foreseeable future as it seeks to corrode international support for Kyiv and waits for Ukraine to run out of ammo.

On the other hand, North Korea is receiving technology to help it advance despite being a pariah state.

A new use for old guns

Old-fashioned munitions have become vital once more in Ukraine, where fighting often resembles the grueling trench warfare of World War I. And with both sides firing thousands of rounds a day, volume counts.

Unlike the sophisticated precision-guided weapons provided to Ukraine by its Western allies, shells don't rely on GPS systems for guidance so can't be countered by electronic-warfare units that scramble their coordinates.

They're something North Korea has in large supply.

"While in most respects the DPRK lags behind NATO states in military technology, mass production of artillery shell ammunition doesn't require sophistication," Jacob Parakilas, a defense analyst at RAND Europe, told Business Insider.

He said that since 1953 North Korea has been preparing for a resumption of an "existential" war with the US, building up massive stockpiles.

The unquenchable need for ammunition in Ukraine means North Korea's stash found an unexpected new value, allowing the North Korean leader, Kim Jong Un, to broker a deal with Russia's President Vladimir Putin that's proving fruitful for both sides.

In return for crates of old-fashioned shells, Kim's getting valuable military technology from the Kremlin.

"Russia can offer some fairly important incentives in terms of technology transfer, which Pyongyang may judge to be more valuable than a portion of its armaments reserve," Parakilas said.

Reports suggest Russia may be providing North Korea with satellite technology enabling it to surveil and more accurately target military sites belonging to the US and its allies in East Asia.

Russia has also used its diplomatic power as a permanent UN Security Council member to shield North Korea, using its veto this year to hobble the commission that monitors North Korea's nuclear program.

Most of the world has long sought to isolate North Korea and pressure it to dismantle its nuclear weapons. The Kremlin's new stance is a U-turn.

Kim may be able to further leverage Russia's need for shells to secure more technology its antiquated military badly needs.

"North Korea's air force, for example, is woefully small and desperately in need of more modern aircraft to be a viable fighting force," Parakilas said.

A precarious alliance

While both leaders are reaping short-term gains, problems on the horizon could derail the alliance.

While North Korea is delivering the shells Russia needs in volume, their quality is often shoddy, and there are doubts about North Korea's capacity to keep up the deliveries.

"Ukrainian sources suggest that the shells Russia has received from the DPRK are dated — some allegedly manufactured in the 1970s and 1980s — and are of poor quality, having a high rate of failure," said Daniel Salisbury, an arms-proliferation expert at King's College London.

And Putin's decision to draw closer to Kim is endangering his relationship with his most important ally, the Chinese leader Xi Jinping.

Analysts told Business Insider in June that Xi was warily watching the security alliance between Russia and North Korea, concerned that it could upset the balance of power in East Asia and spark a conflict China would sooner avoid.

A Cold War alliance renewed

For now, Kim and Putin are reaping the benefits of their relationship, renewing an alliance formed decades ago when the Kremlin helped arm North Korea in its fight against the US and its allies in the Cold War.

"Much of the matériel it has produced during that time will still be at least minimally viable for Russian purposes, since the bulk of its weaponry is Soviet-designed and therefore compatible with what Russian forces use," Parakilas said.