Although the treaty caused difficulties during the First World War, its terms were not changed. Similar problems arose before the Second World War, but Foreign Minister Cordell Hull wanted to preserve the agreement because of its historical importance. In 1939 and 1940, Canada and the United States agreed to interpret the treaty so that weapons would be installed in the Great Lakes, but would no longer be operational until ships left the lakes. In 1942, the United States, now at war and allied with Canada, successfully proposed that weapons be fully installed and tested in the lakes by the end of the war. Following discussions in the Permanent Joint Board on Defense in 1946, Canada similarly proposed to interpret the agreement to allow the use of ships for training purposes when each country informs the other.  Bagot met informally with Secretary of State James Monroe and eventually reached an agreement with his successor, incumbent Minister Richard Rush. The agreement limited military navigation on the Great Lakes to one to two ships per country on each sea. The U.S. Senate ratified the agreement on April 28, 1818.
The British government felt that a diplomatic exchange of letters between Rush and Bagot was sufficient to make the agreement effective. It was perhaps inevitable that an agreement, the technical provisions of which became obsolete more than half a century ago, would from time to time be subject to technical infringements by both parties, and such cases are clearly recorded. However, we believe that it is possible to successfully maintain that, without a certain degree of tolerance, the agreement could hardly have survived until today. But it is a fact of equal importance that even though both governments were forced to deviate from strict compliance with their conditions, they feared that the spirit behind it would be preserved. As far as the size of these vessels is concerned, it has been found that all are loaded with more than a hundred tonnes, the limit set by the agreement. The shift from wood to steel in the middle of the last century, along with other factors, contributed to making this part of the agreement obsolete. To our knowledge, the Canadian government has not objected to the presence of naval vessels on the Great Lakes, with a load of more than one hundred tonnes, and there would be no propensity to question Canada`s maintenance of the vessels currently operating there. For many years, our Department of the Navy seems to have been the practice of deploying on the Great Lakes only “unclassified” ships that have long since survived their usefulness in terms of modern warfare and are no more than fourteen feet deep. I understand that these ships have and are of no other use than the basic training of naval reserves. Lord. Hull believed that it would be desirable to pursue this policy, which went beyond the objectives of the 1817 agreement, but which corresponded so clearly to the current temperament of public opinion.
He informs the Ministry of the Navy. In a confidential letter to the Minister of Foreign Affairs dated January 31, 1939, Admiral Leahy, the incumbent Minister of the Navy, raised some questions about the rush bagot agreement of 1817. Among other things, Admiral Leahy sought Mr. Hull`s advice regarding the mounting of two 4-inch guns on each of the U.S. Navy ships on the Great Lakes, which will be used in the naval reserve formation firing exercise. He wondered if this had been deemed inappropriate, on the possibility of modifying the Rush Bagot agreement to allow this practice. Subsequently, the issue was the subject of informal discussions between officers from our ministries of state and navy. . .