To answer your question: likely Spanish (easiest for native English speakers to learn, widely spoken) and your pick of Russian or Mandarin. Spanish will be most likely used in your everyday life stateside while one of the latter two are widely spoken abroad.
However, I'd like to challenge you on your notion that an invasion of mainland America (or American territories, for that matter) is likely within the foreseeable future. Our closest military peers, China and Russia, are very unlikely to take such a drastic action as starting the third world war right now. Russia is currently being thoroughly embarrassed in Ukraine, which had a relatively weak military that the US is currently using as a proxy to keep the EU's current geographic advantage. If the US is capable of turning motivated farmers currently under threat of losing their nation into a force capable of fighting off a world superpower, what do you think it would be capable of turning the average American in to under the threat of losing their home, livelihood, and rights? I agree the modern US military has lost much of its warfighting spirit - but that's only because we've entered an era of warfare where it isn't needed anywhere other than the frontline, which is historically not on American soil. I'm young, and most of my generation hates overseas wars, but myself and most of my peers would not hesitate to fight a war on American soil against an invading force. It's not difficult to motivate people to fight to win when the alternative is losing their nation - even if it's ideologically divided.
With regards to China, they're hesitant to even invade Taiwan at the moment; why would anyone think they would skip over that and jump straight to an invasion of mainland America? From a purely logistical standpoint, China lacks the sealift capabilities to pull of an invasion of Taiwan, and they certainly lack the logistical capabilities to transport enough troops and equipment to fight the full American MIC on its home turf on a winning timeframe. Chinese forces would have to contend not only with local citizen militias (no doubt newly equipped with gucci rifles, optics, indirect fire and anti-armor capabilities) but also the professional military that would strategically pick its fights. This exact scenario has no doubt been wargamed for literally centuries: every possible invasion route, style, at strategy has likely been predicted and planned for. Any force invading the US would be walking into a slaughter.
Keep in mind that this entire discussion completely neglects the role of nuclear weapons as deterrents. In fact, Russia and China's approach to this topic shows that they are not keen on invading anytime soon; their development of hypersonic missiles to evade US defenses is a show of weakness rather than strength. Why focus on nuclear options that will kill everyone on the planet, thus gaining you nothing from victory, if you could win a ground war with the US?
No doubt nearly every hostile nation to the US has wargames drawn up for hypothetical invasions of mainland USA. But the reality of such an invasion would be so lethal that one should severely doubt whether even a schizophrenic on LSD would entertain the idea.
However, I'd like to challenge you on your notion that an invasion of mainland America (or American territories, for that matter) is likely within the foreseeable future. Our closest military peers, China and Russia, are very unlikely to take such a drastic action as starting the third world war right now. Russia is currently being thoroughly embarrassed in Ukraine, which had a relatively weak military that the US is currently using as a proxy to keep the EU's current geographic advantage. If the US is capable of turning motivated farmers currently under threat of losing their nation into a force capable of fighting off a world superpower, what do you think it would be capable of turning the average American in to under the threat of losing their home, livelihood, and rights? I agree the modern US military has lost much of its warfighting spirit - but that's only because we've entered an era of warfare where it isn't needed anywhere other than the frontline, which is historically not on American soil. I'm young, and most of my generation hates overseas wars, but myself and most of my peers would not hesitate to fight a war on American soil against an invading force. It's not difficult to motivate people to fight to win when the alternative is losing their nation - even if it's ideologically divided.
With regards to China, they're hesitant to even invade Taiwan at the moment; why would anyone think they would skip over that and jump straight to an invasion of mainland America? From a purely logistical standpoint, China lacks the sealift capabilities to pull of an invasion of Taiwan, and they certainly lack the logistical capabilities to transport enough troops and equipment to fight the full American MIC on its home turf on a winning timeframe. Chinese forces would have to contend not only with local citizen militias (no doubt newly equipped with gucci rifles, optics, indirect fire and anti-armor capabilities) but also the professional military that would strategically pick its fights. This exact scenario has no doubt been wargamed for literally centuries: every possible invasion route, style, at strategy has likely been predicted and planned for. Any force invading the US would be walking into a slaughter.
Keep in mind that this entire discussion completely neglects the role of nuclear weapons as deterrents. In fact, Russia and China's approach to this topic shows that they are not keen on invading anytime soon; their development of hypersonic missiles to evade US defenses is a show of weakness rather than strength. Why focus on nuclear options that will kill everyone on the planet, thus gaining you nothing from victory, if you could win a ground war with the US?
No doubt nearly every hostile nation to the US has wargames drawn up for hypothetical invasions of mainland USA. But the reality of such an invasion would be so lethal that one should severely doubt whether even a schizophrenic on LSD would entertain the idea.