So I was reading chapter 13 of Machiavelli's The Prince which talks about the use of auxiliaryforces, mixed ones, and using one's owns.
About auxiliaries he says: "These arms may be useful and good in themselves, but for him who calls them in they are always disadvantageous; for losing, one is undone, and winning, one is their captive."
He talks about Cesare Borgia and his use of mixed forces: "This duke entered the Romagna with auxiliaries, taking there only French soldiers, and with them he captured Imola and Forli; but afterwards, such forces not appearing to him reliable, he turned to mercenaries, discerning less danger in them, and enlisted the Orsini and Vitelli; whom presently, on handling and finding them doubtful, unfaithful, and dangerous, he destroyed and turned to his own men. And the difference between one and the other of these forces can easily be seen when one considers the difference there was in the reputation of the duke, when he had the French, when he had the Orsini and Vitelli, and when he relied on his own soldiers, on whose fidelity he could always count and found it ever increasing; he was never esteemed more highly than when every one saw that he was complete master of his own forces."
He later concludes that the best option is using one's own forces: "I conclude, therefore, that no principality is secure without having its own forces; on the contrary, it is entirely dependent on good fortune, not having the valour which in adversity would defend it."
So the question is: Are there any modern examples of someone needing the use of auxiliary or mixed forces? Was it good in the long run? Is it actually better to use one's own troops nowadays?
There are indeed examples of modern military powers using auxiliary forces. If we disregard "civilian" auxiliary forces, such as the various Women Services auxiliaries employed by many nations during WWI and WWII, the results, much like in Borgia's case, were mixed.
Some examples. In 1878 the British Empire began employing the Khyber Rifles, recruited mostly from members of the Pashtun Afridi tribe, as auxiliaries for the regular Indian British forces. This renowned military force was mostly tasked with security and anti-insurgent roles, and by all accounts performed its role on behalf of Britain very well, in particular in their defense of the eponymous pass during the Frontier Uprising of 1897.
During this war the Khyber Rifles fought their own tribe members during the Afridi uprising, and greatly impressed some of their British commanders. Robert Warburton, who served 18 years as Political Officer in Khyber Agency acknowledged their devotion by saying ‘they had eaten White Queen’s salt.’ Colonel Thomas Holdich acknowledged their devotion in defending the Khyber Rifles at Landikotal and criticized the British response to this particular event; he said ‘Whilst the Afridi fought for us, we failed to fight for ourselves.’ The Khyber Rifle, while proving themselves to be an effective fighting force, nevertheless acted without much consideration to the orders coming from the British, instead falling back on their intimate knowledge of the terrain.
Despite their show of loyalty, this issue with the Rifle was compounded in time. In 1919, during the Third Anglo-Afghan War, the force suffered from low moral and mass desertion, which led to its eventual disbandment. It would be reformed in 1947 and eventually transform into a regular unit in the Pakistani military, serving in its traditional role as the defenders of the Khyber Pass.
For more on them, see Warburton's memoir, Eighteen years in the Khyber, available here: https://archive.org/details/cu31924020369108/page/n207/mode/2up as well as The Khyber Rifles by Jules Stewart, also available via the Internet Archive.
A less flattering example to the performance of auxiliary forces can be found in the Arab Liberation Army and its activity during the Arab-Israeli war of 1948. The ALA was a force comprised of a mix of Palestinians, Syrians and Lebanese Arabs, with a smaller number of Iraqi, Jordanian and Egyptian members. It was formed by the Arab League, both to aid the effort against Israel and to act as counterbalance to the Army of the Holy War, a purely Palestinian militia organized by Abd al-Qadir al-Husayni, a member of the notable Palestinian Husayni family, who the Arab League did not trust. Their numbers at the height of their power were around 6,000 and they were funded by the various members of the League, though Egypt contributed the lion's share of the funds. It was led by Fawzi al-Qawuqji, one of the leaders of the Arab revolt against the British in Palestine in 1936 who was exiled from the region previously and had a notorious feud with Amin al-Husseini, another notable Palestinian leader and a member of the same Husayni family.
From its foundation, the ALA suffered from numerous problems – despite it being ostensibly a force belonging to the League in its entirety it was clear they got their actual orders from Syria, who had little trust in their own military's capability to carry its own weight against the Israelis, and so sought to use a different force, one for which they would not be culpable, especially by the British who had yet to leave the Mandate by the time the ALA began its operations.
This dichotomy between the nationality of the ALA's members, its main funder, its actual high command and the people who actually pulled its strings led to a highly disorganized and poorly trained force which contributed little to the war effort against Israel. The ALA's chain of command was known to exist mostly on the basis of the previous loyalty of any given officer rather than its actual hierarchy. They suffered numerous defeats, first against the various Israeli militias and then against the newly formed IDF. Their most noteworthy efforts in the war were several attempted attacks against isolated Israeli kibbutzim, all of which failed despite the ALA greatly outnumbering the defenders. They were treated with scorn by the rest of the Arab forces and were eventually disbanded in shame by the end of 1948. For more on the subject, I'd suggest the book "The Commander: Fawzi al-Qawuqji and the Fight for Arab Independence 1914-1948", by Laila Parsons.
These are just two brief examples and isn't even touching on the use of mercenaries, but I think it does serve as an example to the problematic nature of auxiliary forces, which did not change much from Machiavelli's time. Their uncertain place in the hierarchy of command often leads to poor communication between the auxiliaries and the main force which employs them, and their different loyalties often override the goal of which campaign they are involved with.