Why did Putin decide to invade Ukraine?

He didn't, he put the troops on Crimea(a peninsula) that before 1957 was Russian(Khrushchev gifted it to Ukraine to keep them happy in the Soviet Union), it never took as Ukrainian though, still mainly Russians live there, and even the Black Sea fleet is still stationed there.

The problem is that the people in Ukraine aren't 1 people, they're truly different Herritage, different Cultural-Backgrounds, which is why the solution shouldn't be rigid, it should be in-line with what the actual People living there want.

I think in his mind he thought that it is the Western forces aka USA, UK AND EU are trying to take control of Ukraine first; a country that is original close to and ally with Russia. He would be seen as too weak of a leader if he just let that to happen.

He is intending to protect Russian tribe in Ukrainian.

Luke 11

21 When a strong man armed keepeth his palace, the things that he possesseth, are in peace.

22 But when a stronger than he cometh upon him, and overcometh him; he taketh from him all his armor wherein he trusted, and divideth his spoils.

He decided to do that for Money, Power, and Territory, just like all wars are fought for. People like to make things complicated; but all wars are fought for Money, Power, and Territory. Obama would like to reassemble the map the way it looked with the Soviet Union. He's a beast, but he's still better than Obama, who wants to disassemble America.

im not sure but its scary, he knows we have a pact with ukraine to protect them because they agreed to give us their nukes if we(USA) agreed to give them protection. I wish we could stay out of things like canada.