BEIJING — U.S. Defense Secretary Jim Mattis, who has accused China of “intimidation and coercion” in the South China Sea, visits Beijing this week as the countries increasingly spar over U.S. arms sales to Taiwan and Beijing’s expanding military presence overseas.

Mattis will be the first defense secretary in President Donald Trump’s administration to visit China. His trip highlights the need for the U.S. and its chief rival in East Asia to engage each other despite increasingly stark differences and mutual suspicion.

Mattis’ mission comes at a difficult time as the Trump administration is set to start taxing $34 billion in Chinese goods in two weeks while Beijing has vowed to retaliate with its own tariffs on U.S. products. The U.S. appears likely to rely on China for help getting North Korea to deliver on denuclearization promises made at this month’s summit between Trump and North Korean leader Kim Jong Un.

A look at some of the thorny issues:

SOUTH CHINA SEA SPARRING

In this April 12, 2018, file photo released by China's Xinhua News Agency, the Liaoning aircraft carrier is accompanied by navy frigates and submarines conducting an exercises in the South China Sea. (Li Gang/Xinhua via AP, File)
In this April 12, 2018, file photo released by China's Xinhua News Agency, the Liaoning aircraft carrier is accompanied by navy frigates and submarines conducting an exercises in the South China Sea. (Li Gang/Xinhua via AP, File)

The South China Sea will likely be near the top of the agenda in discussions between Mattis and Chinese officials, with the U.S. issuing threats against Beijing for its continued militarization of the waters. “There are consequences that will continue to come home to roost, so to speak, with China if they don’t find a way to work more collaboratively with all of the nations who have interests,” Mattis said earlier this month. He said China’s weapons were placed in the region for “intimidation and coercion.” The Pentagon last month withdrew its invitation for China to participate in a large-scale multinational naval exercise in what it called “an initial response” to the militarization of the South China Sea.

KOREAN PENINSULA

In this June 19, 2018, file photo released by China's Xinhua News Agency, Chinese President Xi Jinping, right, poses with North Korean leader Kim Jong Un for a photo during a welcome ceremony at the Great Hall of the People in Beijing. (Ju Peng/Xinhua via AP)
In this June 19, 2018, file photo released by China's Xinhua News Agency, Chinese President Xi Jinping, right, poses with North Korean leader Kim Jong Un for a photo during a welcome ceremony at the Great Hall of the People in Beijing. (Ju Peng/Xinhua via AP)

China was seen as taking home a major win when Trump announced at the summit with Kim that the U.S. would suspend joint U.S.-South Korean war games that North Korea and China have long opposed. Both Mattis and South Korea were seen as caught off-guard by the decision. U.S. and South Korean officials now see the pledge as helping advance nuclear negotiations with North Korea. China welcomed the move, and Kim met with Chinese President Xi Jinping in Beijing last week, though no new measures toward denuclearization have been announced. Trump said last week that “total denuclearization” has “already started taking place.” Mattis, however, was asked whether he had seen any sign that North Korea had begun steps toward denuclearization and replied: “I’m not aware of any. Obviously, we’re at the very front end of the process.”

TANGLING OVER TAIWAN

In this April 18, 2018, file photo released by China's Xinhua News Agency, an armed Chinese helicopter assaults targets with rocket projectiles in a live-fire exercise off China's southeast coast. (Li Shilong/Xinhua via AP)
In this April 18, 2018, file photo released by China's Xinhua News Agency, an armed Chinese helicopter assaults targets with rocket projectiles in a live-fire exercise off China's southeast coast. (Li Shilong/Xinhua via AP)

China’s complaints about U.S. contacts with Taiwan have grown louder, with the U.S. seeking to increase exchanges with the self-ruled island that Beijing claims as its territory, and selling more weapons to it. Mattis will likely hear those arguments made even more forcibly after Trump this year signed the Taiwan Travel Act encouraging high-level visits between the two sides, and the Department of Defense agreed to give American contractors marketing licenses for diesel-electric submarine technology sought by Taiwan’s armed forces. The U.S. government also approved a $1.4 billion arms sale to Taiwan last year, although in an effort to mollify Beijing, has been reluctant to supply everything the island’s leadership wants.

TRUMP’S “SPACE FORCE” CALL

In this June 20, 2018, file photo, U.S. Defense Secretary Jim Mattis, center, steps away after speaking with the media at the Pentagon. (Alex Brandon/AP)
In this June 20, 2018, file photo, U.S. Defense Secretary Jim Mattis, center, steps away after speaking with the media at the Pentagon. (Alex Brandon/AP)

Trump’s announcement last week that he was directing the Pentagon to create a new “Space Force” drew attention in China and may be among the items for discussion in Beijing. Trump framed space as a national security issue, saying he does not want “China and Russia and other countries leading us.” While the U.S. has dominated in space since its 1969 moon landing, China is making strong headway, while Russia is in decline. China says it opposes the militarization of outer space, but demonstrated its growing prowess in the field in 2007 when it fired a missile that destroyed a defunct Chinese satellite, creating an enormous debris field and drawing concerns from the U.S. and others.

CHINESE ARMY’S OVERSEAS EXPANSION

In this April 12, 2018, file photo released by Xinhua News Agency, Chinese President Xi Jinping, center in green military uniform, poses with soldiers on a navy ship for a photo after he reviewed the Chinese People's Liberation Army (PLA) Navy fleet in the South China Sea. (Li Gang/Xinhua via AP)
In this April 12, 2018, file photo released by Xinhua News Agency, Chinese President Xi Jinping, center in green military uniform, poses with soldiers on a navy ship for a photo after he reviewed the Chinese People's Liberation Army (PLA) Navy fleet in the South China Sea. (Li Gang/Xinhua via AP)

In addition to its increased presence in the South China Sea, China’s People’s Liberation Army is expanding its footprint in areas where the U.S. and its allies have traditionally held sway. Most strikingly, China opened its first foreign military base in Djibouti last year, joining the U.S. and a number of other countries with military installations in the Horn of Africa nation. Operating in closer proximity has at times created friction — the U.S. issued a formal complaint to Beijing over the use of lasers that targeted U.S. aircraft in Djibouti on several occasions. The U.S. has also complained about dangerous maneuvers by Chinese military aircraft tailing U.S. surveillance planes in the South and East China seas.

CHINA’S TECHNOLOGICAL THREAT

A man uses his mobile phone in front of a Huawei shop in Shanghai on May 3, 2018. (Johannes Eisele/ AFP via Getty Images)
A man uses his mobile phone in front of a Huawei shop in Shanghai on May 3, 2018. (Johannes Eisele/ AFP via Getty Images)

Department of Defense officials warned lawmakers last week that China and other countries were threatening America’s “technological and military dominance.” Michael D. Griffin, undersecretary of defense for research and engineering, told the House Armed Services Committee’s Military Personnel Subcommittee that China was stealing technology and intellectual property in a way that was “significant and intentional.”