After months of soaring tensions, on Monday Putin ordered his troops to deploy in Ukraine's self-proclaimed republics of Donetsk and Lugansk, hours after recognising their independence.
The scope and the calendar for the deployment are still unclear, but the decisions have fuelled fears of a major escalation in Ukraine.
Russia has massed more than 150,000 soldiers along Ukraine's border, according to Washington and Kyiv, and up to 190,000 including separatists.
Two Wars In Chechnya
In late 1994, after tolerating the Caucasus republic's de-facto independence for three years, Moscow sends in troops to bring Chechnya to heel.
But they meet fierce resistance and are withdrawn in 1996.
They are sent back in again in October 1999 under prime minister Vladimir Putin, who will soon be elected president.
Their mission is an "anti-terrorist operation", in response to deadly attacks in Russia blamed by Moscow on Chechens and an attack by its separatists on neighbouring Daghestan.
In February 2000, the Russian army retakes the Chechen capital Grozny, razing it to the ground with artillery and airstrikes.
The guerrilla movement continues. In 2009 the Kremlin brings an end to its operation, after two wars in which tens of thousands died on both sides.
Lightning Russia-Georgia War
In August 2008, Russia and Georgia go to war for five days over South Ossetia, a small separatist Georgian region.
In the night of August 7 to 8, the Georgian army launches a deadly offensive to retake control of South Ossetia, of which it had lost control since the crumbling of the Soviet Union and war in the early 1990s.
Russia immediately riposts, sending its troops into Georgian territory and quickly inflicting a resounding defeat on the ex-Soviet republic.
The lightning conflict claims several hundred lives.
The Kremlin then recognises the independence of South Ossetia and Abkhazia, another separatist province, and has since maintained a strong military presence there. The West denounces a de-facto occupation.
Conflict In Ukraine
In 2014, after the pro-European Union revolution in Ukraine which forces out the Kremlin-backed leader Viktor Yanukovych, Russia retaliates by annexing the Crimea peninsula.
The annexation is not recognised by the international community.
A separatist uprising then emerges in Donetsk and Lugansk in eastern Ukraine on the border with Russia. They declared independence, unleashing an intense armed conflict with Kyiv.
Kyiv and the West say Russia instigated the eastern uprising and poured arms and troops across the border to bolster them.
Moscow denies the accusations while acknowledging the presence of Russian "volunteers".
The conflict has largely abated since 2015 and is the signature of the Minsk peace agreements.
But, since late 2021, Russia has stepped up military manoeuvres, by air, land and sea, around Ukrainian territory, stationing more than 150,000 soldiers on its border, according to Western estimates.
On Monday, Putin recognises the independence of the two secessionist republics and orders his troops to deploy there.
The conflict in Ukraine has left more than 14,000 dead since 2014.
Intervention In Syria
Russia deploys its military in war-torn Syria in 2015 in support of the forces of President Bashar al-Assad.
Russia's intervention, especially with its airpower, changes the course of the conflict in Assad's favour.
It allows the Damascus regime to clock up decisive victories, seizing back territory that had been lost to rebels and jihadists.
Moscow has two military bases in Syria: the aerodrome in Hmeimim in the northwest and the naval port of Tartus, further south.
More than 63,000 Russian military personnel have deployed to Syria, Moscow says.