BRUSSELS — Whether on the streets of Budapest in 1956, the mountains of Afghanistan in 1979 and again in 2001 or in the swampy forests of Grenada in 1983, invasions have tended to be noisy, unmistakable affairs that screamed their purpose from the start.

After four months of conflict in eastern Ukraine, however, few have chosen to use the “I” word to define the slow-burning war fed by a steady flow of Russian weapons and soldiers across the border.

“I do not want to define it right now, but you can call it what you want,” Chancellor Angela Merkel of Germany told a news conference last weekend in Brussels, where European Union leaders agreed that Russia had increased the “inflows of fighters and weapons” to Ukraine and mounted “aggression” but made no mention of any invasion.

President Obama has been equally circumspect, opting initially for the term “incursion” before denouncing Russia’s “brazen assault” on Ukraine during a speech on Wednesday in Estonia.