Which nation did the Ottoman Empire belong to?
The Ottoman Empire was a nation.
There was no justification for the expansion of the Ottoman Empire because in the 13th to 17th centuries, a nation did not have to justify to outsiders why it was expanding. If a nation, such as the Ottoman Empire, was powerful, it would expand into new regions and improve its internal economy and infrastructure. There is similarly no justification for the expansion of the Hungarian Empire, Polish Empire, or Russian Empire.