Washington (CNN) -- President Obama and Chinese leader Hu Jintao are meeting Monday about nuclear security and other issues. The meeting comes as part of a two-day summit on how to better safeguard nuclear weapons materials and keep them out of the hands of terrorists.

Here's a look at some highs and lows in the sometimes rocky relationship between the U.S. and China.

Highs

• President Richard Nixon sought to improve relations with China in the late 1960s. Observers say a big reason behind the move was to rally the Chinese against the Soviet Union's encroaching power.

Nixon's historic 1972 visit to China, many say, helped improve relations with the communist country.

• China supported the U.S. during the Gulf War in 1991 by deciding not to use its veto power in the U.N. Security Council on the use of force toward Iraq -- and voiced its opposition to Iraqi President Saddam Hussein.

• China and the U.S. have worked together recently in opposing North Korea's nuclear ambitions; China has highlighted its opposition to the country withdrawing from the Nuclear Non-Proliferation Treaty and fears that its nuclear arsenal could threaten the region.

• In November, Obama made a stop in China during his four-nation tour of Asia, meeting with leaders including Jintao. After their talks, the two spoke of a common vision of shared responsibilities and economic opportunities.

Months later, China and the U.S. teamed up to fight climate change and create clean energy. Both leaders said their nations would cooperate to advance technologies and work toward a global agreement on reducing carbon emissions.

Lows

• The U.S. instituted a "containment" policy toward communist superpowers China and the Soviet Union for their involvement in the Korean War in the 1950s.

The countries' providing of support and resources for the North Koreans strained relations with the U.S. It also put an apparent freeze on diplomatic relations with China for nearly 20 years.

• In 1989, China's bloody crackdown on pro-democracy protesters in Beijing's Tiananmen Square drew widespread ire from world leaders, including President George H.W. Bush.

• China's often-criticized human rights record, from religious rights to the treatment of political dissidents, has been a source of tension between the two countries throughout the years.

• In 1996, the U.S. responded with a show of force when China fired missiles at Taiwan. The U.S. support -- and recognition of Taiwan as a separate country -- has created tensions with China, which believes that Taiwan is a part of it.

• In April 2001, a U.S. Navy reconnaissance plane made an emergency landing in China after colliding with a Chinese fighter jet, which was en route to intercept the American plane.

Although the incident was deemed accidental, concerns were raised that the U.S. was threatening the country, a point the U.S. fought.