President Vladimir V. Putin of Russia will travel to North Korea for a two-day visit starting Tuesday to meet with one of the main arms suppliers for his war against Ukraine.As the war has dragged on, Russia has found itself in dire need of conventional weapons, including artillery shells, that North Korea could supply.Here’s some background on what has happened so far and why it matters.What do we know about earlier arms shipments?The United States first accused North Korea of selling artillery to Russia as far back as September 2022, seven months after the war started. At the time, North Korea denied the accusations.Then, last August, the White House warned that Mr. Putin and the North Korean leader, Kim Jong-un, were conducting arms negotiations, and in September, Mr. Kim visited Mr. Putin in eastern Russia. Just weeks later, U.S. officials said that North Korea had shipped more than 1,000 containers of arms to Russia for use in the war in Ukraine. By March, officials said, North Korea had sent close to 7,000 containers of weapons to Russia.If filled with 152-millimeter artillery shells, the containers could carry as many as three million rounds, South Korea’s defense minister said. Or, if filled with 122-millimeter rockets, they could hold more than half a million rounds. They could also have held a mix of both weapons, he said. In his latest estimate, the minister, Shin Wok-sik, last week put the number of North Korea shipping containers sent to Russia at 10,000.Additionally, the White House said in January that Russia had begun launching ballistic missiles produced in North Korea. Arms control experts said that fragments of the short-range Hwasong-11A ballistic missile have been found in the aftermath of Russian airstrikes on Ukrainian cities for months, including in Kharkiv in February. North Korea may also be supplying anti-tank missiles, and portable surface-to-air missiles, as well as rifles, rocket launchers, mortars and shells, South Korean military officials told journalists in November.How important are these weapons?The war in Ukraine is a war of attrition, with Russia and Ukraine trying to outgun each other, firing thousands of artillery shells, missiles and rockets every day. That means that all of the munitions that North Korea gives helps Russia maintain an edge over Ukraine.The Hwasong-11A series missiles, in particular, have a high degree of precision and are difficult to shoot down, experts have said.But at least some of the other weapons are thought to be old or otherwise somewhat ineffective. Last fall, the top U.S. military official at the time, Gen. Mark A. Milley, said he doubted it would be “decisive” when asked if 152-milimeter rounds provided by North Korea would have a big impact on the battlefield.“Would it have a huge difference? I’m skeptical of that,” he said shortly before stepping down as chairman of the Joint Chiefs of Staff.