In modern, industrialised warfare, most countries seem to either wholly or at least partially use equipment sourced from other states. Either these have been purchased new from countries with large arms industries like the US, Russia or the UK, as outdated surplus as seen with how many countries still use mid-20th century soviet tanks and jets, or have been donated to aid in a war effort like with Ukraine.
What I'm wondering is if this is a purely modern phenomenon, or if there was much trade in weaponry in pre-modern times. Would armies in the medieval and ancient worlds have relied entirely on weapons made by local smiths, or would there have been any instances where a country may decide to simply buy weapons to equip it's forces from another country instead?
The example I am familiar with from my own work is the case of the British Civil Wars (1638-1651). In this instance, certainly at the outbreak of the wars, and at the initial stage of expansion in the conflicts, large numbers of weapons were imported, in no small part because the small arms-industry in Britain could not meet demand, and the existing stockpiles were rapidly used up in the huge expansion of forces as warfare broke out. It was only after several years of warfare that the arms industries in the British Isles were approaching being able to supply their forces.
At the outbreak of the wars, and after, the various factions had purchasing agents in various European ports, often France and the Netherlands, seeking to purchase arms, gunpowder, and armour for the growing armies.
Like today, political realities impacted the availability of arms as the wishes of local rulers could impact, if not entirely negate, the exporting of arms to the factions of the British Isles. These were not always a simple decision. For instance, King Charles might expect support from other Royal houses in Europe such as Denmark (his uncle) and France (his brother-in-law) and later on the United Provinces when Charles married his daughter to the son of the Stadtholder. However, not only did these have their own conflicts they were engaged in, but Catholic France might have issues over arms used against Irish Catholic rebels, while the Dutch mercantile community favoured the English Parliament, and the Danes had to remember that Sweden (and to a lesser extent themselves) had close ties to Scotland via extensive Scottish foreign service in the Thirty Years War. When the Scottish Field Marshal Alexandre Leslie requested leave from Swedish service to serve his home country of Scotland, the Swedish Privy council discussed part of his compensation for loyal service be in the form of military supplies including muskets and artillery.
But it was through merchants, rather than heads of state, that the bulk of foreign imports were provided, so a private rather than a state matter (although again, not so simple). This meant that either ready cash, or trusted bills of credit were necessary. The biggest suppliers were merchants in the Netherlands, whose own internal arms industry was impressive. From Amsterdam alone, between Autumn 1642 and Spring 1645, 60,000 muskets, almost 30,000 pairs of pistols, almost 5,500 carbines, 12,000 swords, 16,000 pikes, and vast quantities of other stores were recorded as “licensed exports” to England. And that was just legally declared exports in one port for that period, so the total numbers shifted would have been much, much larger.
I would recommend the book by Peter Edwards, Dealing in Death: The Arms Trade and the British Civil Wars, 1638-52 (Sutton, 2000), but I am happy to tackle any follow up questions you may have.