I believe the solution to all of us "getting along" is in free trade. Should each nation (regardless of ideology) focus on itself, its people, its economy and developing its resources (mineral, agricultural, and tourism) and then extend itself to fairly trade with other nations for what each needs, a co-dependence will develop which would prevent warfare. Capitalism is the solution to everyone getting along, that is provided everyone functions on a fair and level playing field. A global economy and one world governance is evil, repressive, and the antithesis of freedom, independence and sovereignty.

My 2 bits.

Two points here.
In answer to the stated question: NO we can't all get along. History proves that we can't even with the best of intentions.
2nd point. Open borders always leads to a decrease in interior security and lose of sovereignty, always. again history proves this, time and time again.

Not in today's world...Not when the invading culture doesn't or won't speak the language and won't assimilate. Not when the other cultures do not respect the property of others...the most important of which...is the individual.

Diametrically opposed cultures will never mix.

America has absorbed many cultures into One, into One country, One language and One set of laws...it's the only way it works.