The 70th anniversary of the single longest battle in Canadian military history is being marked across the country today.

Thousands of members from the Royal Canadian Navy, the Royal Canadian Air Force and the army gathered Sunday morning at Halifax's Point Pleasant Park to remember thousands of people who lost their lives during the six-year battle.

HMCS Sackville, Canada's only surviving Second World War corvette ship, floated just offshore to lay a wreath and commit to the sea veterans who have recently died. The ashes of 22 naval officers and merchant mariners will be scattered at sea later today.

Defence Minister Peter MacKay was also on hand to introduce the new flag under which all navy ships will sail.

"Sadly, many Canadians died in this valiant effort during the Battle of the Atlantic and never lived to see the freedom for which they fought. But their sacrifice was certainly not for naught. Surely the success of their campaign kept Britain alive and ultimately enabled the victory and liberation of Europe and the defeat of tyranny," said MacKay.

The Battle of the Atlantic was fought between 1939 and 1945. It was one of the defining conflicts of the Second World War.

More than 3,000 sailors and merchant seamen lost their lives delivering supplies across the Atlantic Ocean to Britain.

Halifax served as the home base for the battle.

"This city was the nerve centre for the battle for dominance in this critical mission of the Second World War," said MacKay.

Canada's navy had only six destroyers and 13 ships in total when the Battle of the Atlantic began in 1939, according to a Veteran's Affairs Canada website.

During the next six years, the Canadian fleet grew to 373 fighting vessels, making the Canadian navy the third largest in the world.

This is considered the 70th anniversary of the battle because the allies gained control on the water in what's known as "the turning of the tide" in 1943.

German sub sinks SS Athenia, triggers battle

The Battle of the Atlantic began on Sept. 3, 1939, when a German submarine sank the Montreal-bound passenger ship SS Athenia west of Ireland.

The sinking killed 188 of those aboard, including four Canadians.

The Royal Canadian Navy's chief responsibility during the years-long battle was to escort merchant ship convoys. The first sailed from Halifax on Sept. 16, 1939, escorted by the Canadian destroyer St. Laurent.

By mid-1942, the Royal Canadian Navy, with support from the Royal Canadian Air Force, was providing nearly half the convoy escorts, and afterward carried out the lion's share of escort duty.

Training, air cover, special intelligence and more and better equipment turned the tide in mid-1943, although the battle is considered to have lasted until the end of the Second World War.