In 1880 Belgium celebrated the 50th anniversary of its independence (it recently celebrated its 180th)...This park was built for the celebration.
King Leopold II wished to show the world that the new-ish state of Belgium was prospering. There soon followed Parc Cinquantenaire with its over-the-top, colonially-funded monuments, including the triumphal arch. This was planned for completion by 1880, but was finally finished in 1905, just in time for Belgium's 75th anniversary. Construction work in Brussels is always delayed.
The halls on either side were built more recently as replacements for the original 1880 structures, and one now houses the Autoworld museum, with the military museum opposite.