What might be motivating Russia to once again flex it's muscle's?

Perhaps Russia is flexing it's military muscle once again in response to the US flexing it's military muscle by placing missiles in countries bordering or near Russia. Or maybe Russia sees just how thinly spread the US Army is due to conflicts in Afghanistan, the unnecessary war in Iraq and is looking to take advantage of that. All over the world forces are aligning and plans are being set into motion. Russia, Iran, North Korea...Things aren't looking good.