LONDON — Ireland’s president was welcomed with regal pomp to Queen Elizabeth II’s Windsor Castle home and with thunderous applause to Britain’s Parliament on Tuesday, kicking off a state visit heavy with symbolism for two countries with a tangled and troubled history.

President Michael D. Higgins’ visit is the first by an Irish head of state since Ireland threw off British rule almost a century ago, and a sign of how Northern Ireland’s peace process has transformed relations between two one-time enemies.

Higgins – a veteran left-wing politician, poet and human rights activist – said the two countries had attained “a closeness and warmth that once seemed unachievable.”

Higgins, the first Irish president to address Parliament, said the relationship had gone “from the doubting eyes of estrangement to the trusting eyes of partnership and, in recent years, to the welcoming eyes of friendship.”

A striking bit of symbolism took place at a Windsor Castle banquet hosted by the queen, where one of the guests was Martin McGuinness, once a senior Irish Republican Army commander, now Northern Ireland’s deputy leader. His presence at the queen’s table would have been unthinkable – by both sides – just a few years ago.

More than 3,600 people were killed during a three-decade conflict over Northern Ireland known as “The Troubles.” The main faction of the IRA killed nearly 1,800 in a violent campaign to wrest Northern Ireland from Britain – among them the queen’s cousin, Lord Louis Mountbatten.

Experts say McGuinness was the IRA’s chief of staff when the group assassinated Mountbatten in 1979.

Another 1,000 people were killed by Protestant militants, and about 360 by British security forces.