Norway donates a Christmas tree to London every year. Here's how the tradition began |

LONDON (AP) — London's Christmas tree in Trafalgar Square was lit up Thursday as part of a British tradition with roots in the Norwegian forest.

The tree symbolizes the friendship between London and Oslo in an annual ceremony that highlights the history of the countries' alliance .

Here's how the tradition began:

Token of thanks

The bond between the two countries was forged during World War II . When Nazi Germany invaded Norway in 1940, King Haakon VII and his government-in-exile fled to London. Much of Norway's resistance work grew out of Britain.

The BBC, and its Norwegian counterpart NRK, broadcast the news in Norwegian from London. The reports were a vital lifeline for Norwegians, who listened in secret because of the Nazi radio ban.

As a token of its thanks, the Oslo city