TOKYO >> Japan and China on Thursday mark the 50th anniversary of the 1972 normalization of their ties, but there isn’t much of a celebratory mood. Improved ties between Asia’s two biggest economies are considered vital to the region’s stability and prosperity, but they remain at odds over disputed East China Sea islands and China’s growing military and economic assertiveness in the region.

Here are the key issues in the often strained relations between these powerhouse neighbors:

TERRITORIAL DISPUTES

A huge source of contention is an uninhabited group of Tokyo-controlled, Beijing-claimed East China Sea islands called Senkaku in Japan and Diaoyu in China. Japan insists that the islands, which once hosted a Japanese seafood factory, are part of its territory, both historically and by international law. China says they were stolen by Japan in 1895 and should have been returned at the end of World War II.

The disputed islands are surrounded by rich fishing grounds and undersea oil deposits, and Japan accuses China of suddenly making its territorial claims after the undersea resources were found in a 1969 United Nations report. The 1972 normalization communique did not deal with the issue, but the dispute intensified after Japan’s government in 2012 nationalized the Senkaku islands, leading to violent protests across China. Chinese coast guard and fishing boats are regularly found in the area, routinely violating Japanese waters.

FEAR OF TAIWAN EMERGENCY

Japan, along with its security ally the United States, has openly criticized increased Chinese activities in the South China Seas. Tokyo has also pushed for peace and stability in the Taiwan Strait. China claims Taiwan, a self-governing democracy, and has threatened to annex it by force if necessary.

With a U.S.-China trade war and naval tensions on the rise in the area, Japan is increasingly worried about Taiwan emergencies. China’s increased joint military drills with Russia near Japanese coasts have also irked Japan. Tokyo is shifting its military posture toward southwestern Japan, including Okinawa and remote islands just east of Taiwan.

China staged major military drills in areas surrounding Taiwan in August in an angry response to U.S. House Speaker Nancy Pelosi’s Taipei visit, and fired five ballistic missiles into waters near Okinawa. Fear of conflict over Taiwan adds to Japan’s urgent efforts to reinforce its military capabilities and boost its budget. Japan is currently revising its national security strategy, which is expected to call for the possession of preemptive strike capabilities that opponents say would violate the country’s pacifist constitution.

With Japan’s westernmost island just east of Taiwan, “It is increasingly difficult to see how a Taiwan military contingency would not affect at a minimum the waters and airspace around Japanese territory,” said Amanda Hsiao, senior analyst for China at the Crisis Group.

WARTIME HISTORY

The two countries were at war, starting with clashes in the 1930s, until Japan’s defeat in 1945. Japanese atrocities during the Sino-Japanese war include the Rape of Nanking, the use of chemical and biological weapons and grisly human medical experiments in Manchuria, where Japan’s imperial army had a secret biological weapons unit. Japan also brought nearly 40,000 Chinese laborers to Japanese mines and factories, where many died of malnutrition and abuse.

In the 1972 communique, China waived the right to war compensation, which some experts say was in exchange for Japan’s apology and recognition of China as the only legal government. Japan, however, has provided official development aid totaling 3.6 trillion yen ($25 billion) to China over the past four decades.

YASUKUNI SHRINE

China considers Tokyo’s Yasukuni Shrine — which honors 2.5 million war dead, including convicted war criminals — as a symbol of Japan’s wartime militarism. Beijing views visits by Japanese ministers and lawmakers to the Tokyo shrine as indicative of a lack of remorse over Japan’s wartime aggression. China, along with South Korea, which Japan colonized from 1910-1945, routinely protests against such visits.

ECONOMIC SECURITY

As a top U.S. ally and a major trade partner with China, Japan is in a delicate situation and must balance its position between the two superpowers.

China has been more assertive about pressing other governments to embrace Chinese-led initiatives, including a trade group called the Regional Comprehensive Economic Partnership. Japan, along with the United States, is seeking ways to stand up to increasing Chinese economic influence in the region. Tokyo also wants to reinforce economic security with other democracies in areas such as supply chains and the protection of sensitive technologies, apparently as a counter to China.

Yasuo Fukuda, a former Japanese prime minister who is an active proponent of better ties with China, says friction between Japan and China largely stems from U.S.-China trade issues. “The question is if global trade works better by excluding China,” he said.