Cloning of human have been a taboo even in the late 21st century society, many nations still at odd with each other. Countries like China despite having the most population on Earth decided to clone more soldiers to boost it's frail but growing military prowess, soon many other nations would follow suit. What would be the main political reason for countries to disregard ethical issue and violate such a taboo?
The need to maintain a standing army against a large perceived threat. Space Above and Beyond had cloned/tank grown humans as a counter to rampant AIs going against humanity. In essence national survival trumps all.
A culture where military service isn't seen as a noble profession, especially at the cannon fodder level. Conscription would be the traditional way to deal with this, but the political costs of this would be too high.
"Clones arn't people" legally and culturally so it may be politically more acceptable to use them, especially in a war of offence.
You can bring up clones from the beginning with specific values. Imagine a culture where right from the beginning your country is your father and your mother. You're brought up to fight, kill and die for your country. You're brought up as a mechanic or a rifleman, and effort isn't wasted on teaching you any skills you don't 'need' in your profession.
With genetic engineering you can build in certain traits. Have a safety switch against a uprising by making them require specific supplements to live. Have bigger stronger infantry, leaner pilots designed to handle gforces better. Pick whichever gender does the task better and mass produce them. Disease resistance, and better natural fitness.
Years of television pictures and reports of weeping parents and children and friends of dead soldiers have made it unpalatable to ask or force natural born people to fight in wars. Every time the subject of the military or fighting is brought up, there is huge political pressure against it from the families of people who have lost, or are worried about losing loved ones to battle.
There is no one to cry for the clones though. Previously difficult decisions are easier when it's "Someone Else's Problem" for everyone.
Yes everyone agrees it's a terrible, unethical thing they're doing, and there are probably protests against it organised by people regarded as hippies, lefties and unemployed intellectuals with too much time on their hands, and they have the right to be heard. But there is no outrage, no inferno of emotion. Scenes of parents shouting at, abusing and attacking recruiters while refusing to allow their sons & daughters to be taken away do not occur. No local representatives / councillors / members of parliament who have heard the real life stories and are willing to stand up in the government, not for the clones.
The easiest way to 'justify' cloning for soldiers, is two fold, one you can manipulate their genes to be 'better' soldiers and two, they won't have a 'family'.
It is much easier to send people you don't know into a life threatening situation than your owe friends and family. It would also be easier to make 'claims' about these soldiers making them not quite human. On top of that removing their ability to reproduce would go a long way to removing some of their humanity.
So now you have a country that is sending these 'semi-human' 'super' soldiers into combat against other countries. Why would I as another country be willing to send my sons and daughters to fight these beings? I'll be losing 'real' humans to their 'monster' hoard. So fight fire with fire! We will make our own 'monster' army of 'super soldiers'! We "need" to do it to protect ourselves!
The taboo could come into place for a variety of reasons.
To protect the clones by making sure they don't exist to start with. Sounds weird, but creating a clone could be taboo because you don't do that to clones. Raise a normal child instead.
To protect the rest of humanity from a world where the human genome is used arbitrarily. Creating a clone is taboo because you can't take human DNA for that.
Both reasons could be ignored for military necessity. Remember how major western countries decided to reintroduce torture after 9/11. We could talk about a top-secret supersoldier clone program, few in number and not admitted in public. Or a mass clone army, viewing them as expendable cannon fodder.
How do the clones grow up? If they're raised in normal families, it becomes harder to see them as faceless masses. If they're raised in giant orphanages, it might be easier to ignore them.