Ontario Boundary Dispute
When the Province of Ontario was established in 1867, no defined boundary separated it from the Hudson's Bay Company lands to the north and west. Canada's acquisition of these lands in 1869 raised the issue of provincial and federal jurisdictions and the ensuing dispute was submitted to arbitration. In 1878 a decision favourable to Ontario placed the western boundary at its present location and the northern at the English and Albany Rivers. The federal government rejected the award and in 1881 involved Manitoba by ruling that its eastern limit would be Ontario's still undetermined western boundary. The dispute was settled in 1884 when the Privy Council in Britain upheld the 1878 award.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.