Militarized Great Lakes in the 20th/21st century

The challenge should you accept it is how could the post War of 1812 demilitarization of the Great Lakes either not take effect or not last. More then that how could fleets of warships of the USN vs. RN/RCN face off. What type of warships would be built or used on the Great Lakes? What about coastal fortifications?

Until the St. Lawrence Sea Way (assuming it exists) gets built sending ships via water from the Atlantic to the Great Lakes isn't an option meaning that all but the smallest warships have to be either built on the Lakes or at least have the pieces sent via rail/canal/ Road and assembled on the Lakes.
 
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