NOVOAZOVSK — The trenches appeared as abruptly as the hulking concrete anti-tank obstacles in the road. About three weeks ago, backhoe excavators showed up and began digging tank defense into the dark soil for a few hundred meters on either side of the Novoazovsk border crossing, straddling a major highway running though the Ukrainian-Russian border. Trucks crossing the border here after trekking cargo thousands of miles across southern Ukraine and southern Russia had to begin weaving among the 12-foot high obstacles, which normally serve as artificial breakwaters for the beaches just a few miles away.