What are the risks in uniting people [in war] against a common enemy?

Answer 1

Many leaders (mostly dictators, but also some demagogues in democracies) have used war against a common enemy as the means to unite their people behind them.

The big risk in this survival strategy lies in the nature of war itself. War is a risky business. You might lose. And, if you do lose, your own position as a leader will be weakened.

Examples of this are countless. To give just a few: -
[1] Saddam Hussein tries to take advantage of Iran's presumed weakness in the wake of the anti-Shah revolution, and to unite his very disunited Iraqis in a glorious war of conquest. Result? A million or so dead, and Iraq much weakened.
[2] Saddam Hussein tries to unite his even more disunited Iraqis behind him with an easy-victory parade into Kuwait. Result? Saddam has his butt kicked in Desert Storm, faces a major revolt in his Shiite southern provinces, and permanently loses control of the Kurdish northern provinces.
[3] Idi Aman Dada, the mad and murderous dictator of Uganda, invades Tanzania in the belief that this will unite the tribes of Uganda behind him. Result? The Tanzanians (helped by rebel Ugandans) kick his butt, occupy his capital, and Idi Aman has to flee into exile.
[4] Hitler ... (enough said).
[5] Mussolini ... as for Hitler.

Answer 2

For a good example of the risks of uniting a people (in war) against a "common enemy", see Adolf Hitler, the NSDAP, and the Second World War.

If that doesn't ring any bells for you, then you may realize Germany did NOT have a common enemy in England ( and her Dominions) and the U.S.A.