By the 1880's a dramatic revolution in naval and military technology made changes in the Halifax defence system essential. Steam propulsion, powerful breach loading guns and the advent of the motor torpedo boat transformed naval warfare. Defence schemes established for years became obsolete. Britain's overseas empire seemed threatened. A metamorphosis in British defence policy and coast defence strategy was the result.

The Royal Navy bore the brunt of responsibility in the new scheme. The Naval Defence Act of 1889 adopted the "Two Power Standard" whereby the strength of the Royal Navy would be at least equal to that of the combined fleets of the next two largest navies. To be effective, however, the fleet had to be concentrated in home waters. The protection of exposed overseas ports, such as Halifax, would be the responsibility of improved coastal fortifications.

The home authorities decided to strengthen the advanced line of defence for Halifax. A new fort with long range breech loading guns was planned for the southern end of McNabs Island. Construction of Fort McNab began in 1888 and was completed in 1892. The armament included one 10 inch breech loader (BL) and two 6 inch BL's. The effectiveness of the new guns was increased by the installation of an elaborate fire control system.

Since Fort McNab was built at a time of rapidly changing technology, it is not surprising that major renovations were needed within a dozen years of its completion. The main changes involved the guns and their mounts. More powerful guns on central pivot mountings were introduced by 1906. The included one 9.2 inch BL and two 6 inch BL's. By this time, however, Britain had concluded that war with the united States was only a remote possibility and that her forces at Halifax could be withdrawn. The Canadian Government agreed to take over all the defences, including the newly renovated Fort McNab.

As the diplomatic situation in Europe deteriorated, plans were developed to defend Halifax against enemy action. An Examination System was established to control marine traffic into the port. The decision to make Fort McNab responsible for the examination anchorage necessitated certain changes. Searchlight emplacements were constructed in 1914 to enable the fort to carry out its additional role more effectively. A 6 pound QF gun was mounted on the parapet of the fort to act as a "bring-to" gun. Throughout the war Fort McNab also remained one of the more important counter-bombardment batteries in the Halifax defence system. Substantial improvements were made to the living quarters of its large garrison. All this activity came to an end with the signing of the Armistice. The examination battery remained partially manned until January of 1919 when the fort was officially closed down.

In the interwar period Fort McNab was neglected, as was the entire defence system. While all realized that Fort McNab's armament was no longer powerful enough to deal with a modern threat nothing was done to improve its effectiveness. The only important construction during these years was the replacement of temporary wartime buildings with more permanent structures. The old 9.2 inch barrel was replaced in 1921 but nothing else was done. By 1932 retrenchment hit the Halifax defences with full force. Fort McNab was one of the forts closed down by the department of national Defence. In effect, the fort was put in moth-balls.

All of this changed with the deteriorating political situation in Europe. In 1937 a complete re-assessment of the coast defence was undertaken by Major B.D. Treatt, R.A. His major recommendation was the construction of two new powerful counter-bombardment batteries at Hartlen Point and Chebucto Head. Fort McNab's 9.2 inch BL was to be transferred to one of the new forts, while its 6 inch battery was to be retained for "close defence" and examination purposes. New electric lights were to be provided for the fort. Over the next two years a general re-allocation of coastal guns in Canada took place. McNab's 6 inch group were sent to the West Coast and were replaced with 6 inch guns from Quebec City. The installation of the guns marked the return of Fort McNab to operational status. By September of 1939 it was ready, but just barely, to assume its role in the protection of Halifax.

Fort McNab had three main roles during World War II. First of all it was a counter bombardment battery for close in defence. Secondly it controlled the searchlights on the southern end of the island. Thirdly, Fort McNab oversaw the examination anchorage. Many changes occurred at Fort McNab during World War II. A new Battery Command Post was constructed in 1940-41. As planned the 9.2 inch BL was removed in 1942 and shifted to Hartlen Point (Devils Battery). A year later a 75mm gun was mounted to replace the 6 pounder as the examination or "bring-to" gun. In 1944 the 6 inch group was replaced with more modern 6 inch guns from Sandwich Battery. The final major improvement to the fort during the war was the construction of a radar post in 1945.

When the war ended Fort McNab was entirely dismantled and placed in "heavy maintenance." By 1948 the Department of National Defence was having second thoughts about the closure and ordered its reactivation. The final armament change occurred in 1953 when a 4 inch twin naval gun replaced No. 2 6 inch gun. The fort operated until January of 1960 when its was dismantled and closed for the last time.

Fort McNab was the first of the breech loading forts to be built in Halifax. It represents a technological breakthrough in the science of coastal defence. According to one authority it is a "textbook example of the new style of coast fortifications" adopted by the Royal Engineers in the 1880's. Even more than this, Fort McNab represents an evolution of coast defence technology from the 1880's to the 1940's. Advances in ordnance, detection and fire control are all represented at the site. Among its more important extant resources are the largest breech loading emplacement in Halifax, early position finding cells (sometimes more generally called range finding cells), an extensive magazine complex, searchlight defences and a radar post. Taken together these resources present a unique microcosm of coastal defence evolution from the 1880's to the 1940's.