what are the odds of someone invading the US?

Slim to NONE, of course. Invasion is the most costly way to wage war, especially on the other side of an ocean. D-Day in WWII, was only 20 miles or so across the English Channel to get to Normandy. Think of the logistical problem of a couple thousand miles. Look at Pearl Harbor. The Japanese failed to catch their primary targets, the aircraft carriers. The knew from day one that war in the Pacific would be mainly an air war. The battleships were simply secondary, convenient targets of opportunity. The military machine of the USA is huge. Anyone invading the homeland would initially find it VERY easy to overrun unarmed civilians. But, soon after, the war machine would get into gear and defend and then attack to drive back the invaders, simply of the numbers that would BE available after a small amount of time! And then, the invaders would be driven back and face a no retreat situation with their back to the sea. Dunkirk was such a situation for the allies in WWII. The British called on every craft available to evacuate the armed forces from Dunkirk, and there were a LOT of small boats available, carrying 5 or 6 soldiers at a time. Germany thought they had time to mop up the Allied forces, thinking wrongfully, that the armed forces were trapped on the beaches after being defeated on the field of battle inland. Again, we're only talking 20 miles of open water or so, not an entire ocean. It all comes down to the logistics of SUPPORT and SUPPLY LINES. Our military only has to mobilize locally, down the street, while an enemy would be working at thousands of miles by remote control. Canada and Mexico may not have much to offer compared to the industrial resources the USA has to wage war, but I am certain they would do what they could as allies, knowing that the downfall of the US would also mean theirs soon thereafter. If they did not believe this, we would not have so much of a problem with illegal border crossings. There was a movie a while back, "Red Dawn" which addressed exactly this scenario, with the USSR invading the USA. It was unrealistic then, and it is unrealistic today. The movie NEVER addressed the logistical problems of the invading forces in a conventional war. During the "cold war" the fear was the infrastructure would be destroyed by a "first strike" by the USSR and our response was if you hit us, will will hit you back even harder. Mutual Assured Destruction (MAD) was the term for hitting back harder and why we have SO many nuclear warheads and so many missiles to throw them at an enemy who launches first. Our first line of defense was the DEW line, a series of radar units looking for missiles rising on a ballisitic course over the pole, which would warn us so we could launch a retaliatory response before the first bomb hit us. Today, satellites look for rising heat plumes from the rocket exhaust, which is much more reliable than radar over the horizon. The key is the ability to track the missiles to know where they are going. Every day, the system is fooled by meteors leaving a heat plume in the upper atmosphere. For a rather humorous look at the whole scenario, watch the movie, "The Russians are coming! The Russians are coming!" I was on the front lines of the cold war, in a fast attack submarine. Our job was to protect the ballistic missile subs so they could make the retaliatory launch. OK, so I veered off topic a bit, but for ME, the whole of history I find most interesting, especially since I personally, lived through part of it...

USS Permit, SSN-594. Fast attack TOUGH!

Admiral Yamamoto of the forces that attacked Pearl Harbor is reported to have said, when informed of the results of the attack, words to the effect that all they had succeeded in doing was to wake a sleeping giant... These words remain as true today as they were THEN...

Its no longer a question of "can it happen?". It is happening. America's "Third War" is one with illegal immigrants crossing north into Texas, NM, Arizona, and California. At one point, Army National Guardsmen were activated to increase military presence in border towns. When Cartel leaders are openly targeting law enforcement, as well as military and city officials, and then conducting raids on small, vulnerable border towns, I would call that an invasion. Something similar happened right around the turn of the century back in 1900. General John J. Pershing told the defunct mexican government at the time (not unlike the present) to piss off; he led an army unit into northern mexico and rooted out the mexican guerrilla units. The raids immediately stopped. If the mexican government cannot govern their northern states, we can help with a little bit of force...

Its possible but very unlikely, unless its a unified attack from several nations no one nation can take us on. Many people have played with the idea of Russia or China invading the US but the fact is they are just far too technologically inferior compared to our military not to mention even if they do manage to beat back our military we still have the 2nd Amendment. Unlike other Western nations our people are armed to the teeth and know how to use their weapons, so if anyone tries to invade the trees will literally be shooting at their troops.

No individual country would invade us. It'd have to be multiple ones at the same time from different directions. China would be more likely to go to Cyber Warfare than physical conflict. And they've hit us hard off and on for the past 5 years just it's rare when the Gov will ever let it get out to news agencies.

We already ARE being invaded. They just aren't carrying guns (well, most aren't). Their main weapon is our own government. Yes, I am referring to illegal immigration.

Physically? 0%

Through cyberspace? 100%

100%

Just south of nil.