Passage: Within a year after the first trade in guns, Japanese swordsmiths and ironsmiths managed to reproduce the matchlock mechanism and mass-produce the Portuguese guns. Barely fifty years later, "by the end of the 16th century, guns were almost certainly more common in Japan than in any other country in the world", its armies equipped with a number of guns dwarfing any contemporary army in Europe (Perrin). The guns were strongly instrumental in the unification of Japan under Toyotomi Hideyoshi and Tokugawa Ieyasu, as well as in the invasions of Korea in 1592 and 1597.  The daimyo who initiated the unification of Japan, Oda Nobunaga, made extensive use of guns (arquebus) when playing a key role in the Battle of Nagashino, as dramatised in Akira Kurosawa's 1980 film Kagemusha (Shadow Warrior).

Question: What was a strong unification tool between the nations and invasions?
guns