By Rupert Wingfield-Hayes BBC Beijing correspondent

When US President George W Bush steps off Airforce One at Beijing's international airport it will be exactly 30 years to the day since President Richard Nixon did the same, becoming the first US President ever to visit Communist China.

For China it will be a symbolic moment, a vindication for their policy of engagement with the United States, a policy that has at times faced fierce opposition.

As President Bush arrives relations between the two countries are on a high, bolstered by China's strong support for America's "war against terror".

Major divides

President Bush will lavish praise on President Jiang Zemin for that support. Their joint news conference will be broadcast live across China, giving Mr Bush a chance to speak directly to the Chinese people.

Richard Nixon during his historic visit 30 years ago

And like Nixon before him he will find time to stroll on the Great Wall.

All warm and fuzzy then?

Well perhaps not quite. Behind the outward show of warmth there are major issues that divide these two great powers as deeply as ever.

President Bush's "Axis of Evil" is one. China is extremely concerned that having finished off the Taleban, America is now intent on turning its military might on Iraq.

Beijing is also disturbed by the inclusion of North Korea in this new "Axis". President Bush will be told that North Korea should be engaged rather than threatened.

For President Bush the top issue on the agenda is proliferation, specifically China's continued supply of sensitive missile technology to countries like Pakistan and Iran.

Christian issue

President Bush wants China to give a clear commitment to end such exports. China has made such promises before, but this time America wants it in writing.

Mr Bush is also reported to be deeply unhappy about the treatment of China's millions of Christians. As a born-again Christian himself its a very personal issue for the US President.

The spy plane incident soured relations

He is expected to push China hard to recognise the Vatican and the Pope's leadership of the Catholic church, and to legalise the tens of thousands of underground church groups across China that are currently forced to practice outside the law.

No US-China meeting can pass without some reference to Taiwan. For China it remains the most difficult and sensitive issue in its relationship with Washington.

Beijing bristles at America's support for the island China considers a part of its own territory.

One of Mr. Bush's first actions on becoming president last year was to grant Taiwan the largest US arms package in 20 years. Beijing was incensed.

Mistrust

It says such military support for Taiwan only encourages the island to move further towards formal independence from China, something Beijing has made clear it will not allow.

America responds that its only helping Taiwan defend itself against a growing military threat from China.

In the 30 years since Nixon came the relationship between China and the US has matured.

But there are still huge areas of mistrust and potential misunderstanding. As one Chinese diplomat recently said of the two countries "We are no longer enemies, but nor are we friends."