There has been some silliness from those who should know better in seeking parallels from history to the nascent Anglo-French military alliance; the Crimean War is an utterly absurd comparison, and even comparing WWI and WWII is pushing reasonable limits. The more recent UN effort in the Balkans also misses the mark - sorry, lads, this really wasn't a war. No, I think the most recent example of a true war in which international forces came under various combinations of national command is the Korean War, in which the nations of the UN faced China.
In November 1950 the Chinese army was smashing the US 8th army, which was in full retreat back to the coast. Facing the Chinese 38 Corps as a rearguard holding a critical road junction was the tiny Turkish Brigade, just three infantry battalions with a few borrowed tanks. The stubborn resistance of the Turks allowed an entire US division to escape. Finally, facing two entire Chinese divisions, the tiny Turkish brigade was effectively destroyed; out of ammunition, they fought savagely with fists, rocks, trenching tools and the bayonet, taking a terrible toll of Chinese. But they saved the day. The Battle of Wawon stands still as an example of the potential of a mixed force on the battlefield. Brig. Tahsin Yazici, the Turkish commander, had fought at Gallipoli and displayed the same tenacity in Korea. When ordered to withdraw by his American superior, he replied "Withdraw? Why withdraw? We are killing lots of them". If given the choice of who you'd like protecting your flank, the Turks must surely be high on the list.
The UN force left dead from a galaxy of nations on the battlefield; US, UK, Turkey, France, Australia, New Zealand, Canada, Greece, Colombia, Thailand, Philippines, South Africa, Netherlands and Belgium. Even the tiny Luxembourg contingent of 44 men lost 2 KIA. Command structures were an international mix, but it worked; soldiers are soldiers the world over.
The Korean war is often forgotten these days, but as you wear your poppy spare a thought for the brave young men of this country, many of them national servicemen, who fought and died on that ice-blasted peninsula sixty years ago.