Short answers, pulled from the story.
The Union State between Belarus and Russia was established on the 8th of December 1999. It was the culmination of an integration process launched on the 2nd of April 1996, which had also produced the Union of Belarus and Russia exactly a year later in 1997.
Russia values Belarus primarily for its geographic position as a western neighbor sharing a land border, its role as a transit corridor for Russian energy exports, and its function as a military buffer. After NATO expanded eastward and color revolutions spread through the former Soviet space, Belarus became increasingly critical to Russia's security posture.
The 2004 Russia-Belarus gas dispute began when Gazprom, seeking to acquire the Belarusian transit network operator Beltransgaz, cut off gas supplies to Belarus on the 1st of January 2004 after price negotiations collapsed. A new gas contract was signed in June 2004, which restored supplies.
In July 2020, Belarusian authorities arrested 33 Russian military contractors in Minsk. Lukashenko accused Russia of trying to send 200 Wagner Group fighters into Belarus to destabilize the country before the 9th of August presidential election. Russia's security chief Dmitry Medvedev warned Belarus to release the contractors on the 5th of August 2020.
Yes. On the 25th of March 2023, Russian president Vladimir Putin announced that Russia would station tactical nuclear weapons in Belarus. Belarus had previously stated that Russia could bring nuclear weapons onto its soil following Russia's invasion of Ukraine in February 2022.
As of 2009, Russia accounted for approximately 48% of Belarus' external trade. Belarus, by contrast, accounted for around 6% of Russia's total trade, illustrating the asymmetric economic dependence between the two countries.